Most of you know that I have been an avid Glock shooter for many years.  I've become quite comfortable with the Glock grip angle and overall size and shape of Glock's grips.  Last March, in celebration of the 1911's  100th anniversary, I purchased my first 1911 since the Army 40+ years ago.  I never added a 1911 to my collection because as a young soldier with long thin hands, I found the 1911's grip somewhat difficult to grasp securely as it was too thin for me to grasp comfortably.  Fast forward 40+ years and now I find myself with more than a few pounds on my frame and meatier hands.  I found I could grasp the 1911 much better and hence my decision to acquire a 1911.
As I practiced with my Springfield, I realized that I was starting to have issues with my grip on my Glocks because of the basic difference in grip and grip angle between the two platforms. At first  I felt that it was a matter of training to switch from one style to another, however, I also noticed that upon presentation, my Glock's were starting to point muzzle high and I was not able to 'train' them back down.  Try as I might, I found that switching between the different grip angles was difficult to do quickly and accurately.
Recently, I noticed some discussions about Grip Force Product's Grip Force Adapter (GFA).  As I read about it, even though I have never had any 'slide bite' issues, I concluded that it might be worth a try because they changed the grip angle so I called and spoke to Frank, the owner.  He explained that he had updated the advertizing announcing that the GEN 123 version (the correct one for my gen 3 Glocks) also fit the large frame Glocks (G20 & 21) because their backstrap radius was the same as the standard frame Glocks.  After our discussion, I decided to purchase two, one for my G20/21L and one for my G24 and give them a try.
Installation and Fit:
When they arrived, I inspected them and found them to be well made and of a style very similar to the Glock's backstrap.  Included with the GFAs were extra extended Trigger Housing Pins; one of which is needed to hold the GFA in place.  After viewing the installation video on his website HERE, I installed them on my two Glocks under the Hogue HandAll grips both already wore.  They installed quickly, easily, and securely especially under the HandAll.  
The first thing I noticed was that they fit my Glock's backstraps very well, felt natural, and the added beaver tail made the chance of a 'slide bite' virtually impossible.  Second, I noticed that the length of pull (LOP is distance from the backstrap to the trigger) was increased somewhat which I really enjoy because of my long fingers.  Third, I found that upon presentation, my Glock's muzzles were back down in line with the rear sights.  While Grip Force Products encourages purchasers to modify their product anyway they choose to help custom fit them to the individual shooter's grip, for this initial go I elected to leave the units stock and shoot my Glocks a few times before I decided where I wanted to customize them.  
Grip Angle Difference:
As is evident in the pictures below, the GFAs significantly changed the upper grip angle on the Glocks to an overall grip angle not much different than the 1911's.  Gone is the 'hump' at the base of the grip because the 'hollow' above it is now filled in.  This creates a noticeably different feel and as I mentioned above, it tends to push the muzzle down which improves the natural pointing for those who shoot other brands of pistols with grip angles more closely shaped to the 1911.
My G24 and G23 (ccw piece) comparing revised backstrap angle with the GFA installed under my HandAll:

My G20/21L and G23 (ccw piece) comparing revised backstrap angle with the GFA installed under my HandAll:


Change in Grip LOPs:
Concerning the LOP increase.  As I said, I have long fingers and I've wanted an increased LOP on my Glocks.  The GFA did increase this dimension a bit which makes for a better trigger feel on my pistols.  On GFP's website under the 'Technical' Data' page, they state that the GEN 123 adds only ".077" (seventy-seven thousandths of an inch or 1.96 mm)" to the LOP on my gen 3s.  Compare the extension of my trigger finger as well as the tip of my thumb vs
the 1st knuckle of my middle finger in the pictures below to see the difference in LOP the GFAs
add on my pistols.

My hold on my stock G23 grip without a Hogue HandAll or the GFA:

My hold on my G24 with both a Hogue HandAll and the GFA installed.  Note the slightly longer LOP on these same size (40s&w) frames:

My hold on my custom G20/21L grip with a Hogue HandAll but without the GFA:

My hold on my G20/21L with the GFA installed under the Hogue HandAll.  Note the slightly increased LOP:

Added Bonus - An Indexing Point:
In addition to the above, I noted a significant benefit from installing the GFAs with respect to grip height and grasp repeatability.  The GFA has a built up area around the Trigger Housing Hole attach point on each side to engage the Trigger Housing Pin.  This area, like the entire GFA, can be filed down to suit the individual shooter during custom fitting, however, as I said above, I decided to try the GFAs as delivered prior to making any adjustments on them.  I'm glad I did because I noticed that the small bump on each side of the grip caused by the attach point actually guided my hand up higher on the grip.  It's in effect a 'grip placement aide' that naturally encourages you to raise your grip to the better, more controllable position because you can feel the small bump if your grip is low.  For my hand shape, the bump is a serious benefit to ensure proper, repeatable grips and it will stay in place on both pistols.
Performance At The Range:
   
All the do dads in the world aren't worth a nickel unless they actually help you at the range so while I liked the initial fit, a trip to my range to 'put rounds on target' was mandatory to fully evaluate these GFAs.  So, yesterday afternoon I packed up both Glocks and went for some shootin'.  As I'm comfortable with my Glocks from years of shooting them, I found that the grip 'feel' was basically the same as before, however, the attachment point humps 'encouraged' me to grip up high on the grip; just what I want to do.
I tried my G21L version (45acp) first with some 230grn 'warm' loads and my first magazine was a pleasant surprise.  As soft as the Glock shoots 45acp, the GFA installed helped my 'back on target' motion as I needed less push to get the sights back in line because the modified grip angle was causing the muzzle to naturally lower from that on a stock grip now that I was also shooting my 1911.  Then I swapped out the upper and tried it again as a G20L (10mm) with 165grn hot loads and once again the 'back on target' motion was improved.
Now, back to the G21L configuration to see how my shooting performance was affected.  I placed a few bowling pins on our 75 yd berm and shot both slow and rapid fire.  Next, I switched back to the G20L configuration and repeated my shots at another bowling pin on the berm.  I'm pleased to say that with both calibers, my shots grouped surprisingly well with multiple hits on the pins and the rest near misses.  I also tried my G24 with similar results. BTW,  I like to test out my shooting a longer ranges as I subscribe to the 'aim small, miss small' branch of shooting and I've found that shooting over 50 yards highlights my technique and when I'm getting sloppy or lazy.
Conclusions:

I'm well pleased with the GFAs and they both will stay on my Glocks.  I will need to shoot them a few more times to really get an idea of where, if anywhere, I'll decide to shave them down a bit.  I've already decided that the lower edge of the unit where it blends into the Glock's grip could use a bit of thinning the remove the transition hump but at this point.  It's not a big thing but I feel that a bit of tuning at the transition is in order at this point.  Further, I don't see any really glaring areas that need attention but I'll leave that decision to a later date after I'm more familiar with them.
So, in summary I believe that Grip Force Product's Grip Force Adapter is well worth consideration.  If you find that you're having to force the muzzle down to acquire the correct sight picture, if you want to change your Glock's grip angle to more closely match the 1911's, if you sometimes get 'bit' by your slide, or if you're looking to slightly increase the LOP on your Glock I think that the Grip Force Adapter is a legitimate addition that's appears well made and is reasonably priced.  I'm even considering adding one to my G23 CCW piece.  
Note:  Lastly, this review was of their black Gen 123 adapter only as both of my Glocks are black gen 3s.  Grip Force Products also produces them in colors as well as an adapter for gen 4 Glocks and I can only assume that those work as well as the ones I tested.  Further, I do not work for or with Grip Force Products nor have I received any consideration for this review.  I undertook this review solely for my own understanding and to share with you all my observations without any discussion with Grip Force Products.  In fact, Grip Force Products will get their first indication that I even reviewed their product if and when they read this thread.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

I have the GFA on a 30SF and a 19. A worthwhile addition IMHO
Easy installation and very comfortable.

Nice write up!

I agree with all of your findings, pointing,  grip, l.o.p. and fit.

I also find that a nice blend job of the ribs smoothing them out at the end of the bevertail and down toward the arch of the backstrap make a high grip more comfy no more tracks from the slide or the grip in the web of my hand.

you may have just convinced me to try one.