I just picked up a Faxon Gen2 bolt and a Kaw Valley (Faxon) 4.5" 9mm barrel.

Not interested in a Promag block, but the KAK looks promising.  If I have to step up to a Hahn to get reliability, then a dedicated 9mm lower is actually cheaper ($175 vs $200 for the Hahn).

Has anyone here used the KAK block?  Lots of good reviews on KAK's site, but a few guys mention having to tune the gun a bit.   Or is a dedicated 9mm lower with the mag block machined out and integrated the proper way to go?

Advice welcome.

I use the Hahn mag block.  I have a number of lowers with aftermarket triggers in them(2 1/2lb) and by the time I spend money on a dedicated lower plus the $200-300 trigger might have well have the option of using it as a .223 or a 9mm.  Only takes a minute or two to remove the mag block. Plus I wanted to use the metalform (Colt) mags.  Don't own a Glock....have no intention of every buying one.  I now have three of these built now....and they all run great.  Bought 25 mags when PSA had them on sale and I'm set.  I've been shooting them in USPSA matches for two seasons now.  Couldn't be happier!

Ghost firearms offers a mag block for $80.00. It looks good. No experience with it.

I'm another happy Hahn block user.  The gun has been great.  Only trouble I had was with some CProducts mags that needed some modification to the follower so the bolt would lock back on empty.  Some Metalform mags had issues with the BHO tab, but that wasn't the guns fault.

Ive been running with the KAK block and have been very happy with it. I did not have to fine tune anything to get it to run reliably. I wouldn't hesitate to buy another.

I believe the KAK bottom load block is the same one as designed by Rudy at Macon Armory... I bought mine directly from Rudy, but the KAK is cheaper... I have the mag block for my registered SBR, but shoot a dedicated New Frontier Armory billet lower (Glock mag) for USPSA PCC...I already shoot a G34 for competition, and had the mag pouches and extra mags so the choice was easy...I don't have a need for last round bolt hold open... the Colt style mags for the mag block are relatively cheap... I have considerably more shots on the Glock billet lower than the SBR... if you want a range toy that looks like a WWII sub gun, then the straight stick Colt looks cooler... I like the direct into chamber off the single feed Glock magazines for competition

I don't want to speak for the great JeepCJ87 or step on his/her toes, but I believe all the info you seek has already been discussed a multitude of times and basically comes down to personal preference.  Some people prefer the look of a dedicated lower or think they work better.  Others prefer the flexibility/options of a block and lower.  Others prefer the Glock mag setups which are currently only available as dedicated lowers.

You can get Hahn dedicated block from Brownells for about $160 with discounts/sales/coupons.  The KAK block is $100 plus shipping.  I have both and they work pretty much the same with my setups and the ammo I use.  Ghost firearms appears to be reselling the KAK block for $89. (compare pics at KAK and Ghost - they are the same block)

As far as tuning, if you build a 9mm AR from components, there is going to be a minimum amount of basic tuning that will be necessary and it will depend on your specific setup and ammo.

I have a PSA Glock lower, it is great and worth the money.
But I also have a 5.56  SBR, that I use the torkmag  G block system on and it also works
and its only around $70. It changes out in a couple of min and you don't have to take out the mag release.
 Well worth not getting another SBR frame

Good call on the Promag block.  I had one that came with an upper and it was a complete POS.  With their reputation for lousy magazines, it came as no big surprise.

I'm using the RRA block. So far, so good. It's mid priced among the offerings.

I waited to get into the 9mm AR world until Rock River introduced their dedicated 9mm lowers. I don't want to have to fiddle around with my gun to get it to run.

I’m going to go with a Hahn block, I just need to decide too or bottom insert? I’m leaning top insert. I have a factory Colt I’m going to sell for the simple fact I have a SBR lower that will make a great host, and if I SBR the Colt it’s still 200.00, plus this way I can shoot now.

I use the Hahn dedicated block.  It's secured by the bolt catch pin.  So it's permanent in the sense that it needs tools to be removed.  If you can assemble/disassemble a lower, you'll have no trouble installing or removing it.

Spikes 9mm Colt for $150

Good to know, when I saw permanent I never even bothered to look at it, as I assumed drilling and using pins. Being that it’s already a NFA SBR lower the last thing I want is to permanently alter it, especially to a 9mm. Thanks for the info.

Not anymore

See the other thread(s) on the new offering from Stern Defense - Glock mag mag block.

At the same cost as a lower from PSA, the only advantage I can see to it is if you have a tax stamped lower of some type (FA or SBR)