Other than it being geewhiz cool...  How reliable are they?  Can they easily be knocked out of place?  I can see with a flap top target rifle so that the sight does not obscure the sight picture on a scope, but with a tactical carbine?  Is there a real purpose or just another sexy bell and whistle?

the one i've used (GG&G) is very reliable. mine is the older one that doesn't lock in the 'up' position, but the new ones won't knock down.
actually, on a target rifle with a scope, the front sight is less likely to get in the way than with a tactical carbine with non-magnified optics, like the aimpoint and eotech.  using those optics, i prefer having the front sight down, although it doesn't bother most other people anyways.  just my personal preference.
cheers,
MM

Neither a bell nor a whistle, just money out of your pocket.  The front sight doesn't get in the way of dot sights or optics, so why remove it or flip it.

KISS (Keep It Simple, Stupid) is my guiding primciple with weapons.  If yours isn't a weapon then add all the gimmicks you want [;)].

-- Chuck

I agree with Chuck given what your needs are.  If it ain't broke, don't fix it.
I am very shy when it comes to aftermarket "bolt-on" flip front sight units, as it is also your gas block.  So when the front sight tower gotta go, I hacksaw it off and round-over the top, but it stays on the barrel.
If I want a flip iron sight up front, I use one of our KAC units that attaches to the top rail of the RAS.
In the military, however, I think its the up and coming thing.  First off, they will buy these as factory units so the gas block portion of the front folder is "married" to that barrel.  Secondly, a new generation of "in-line" clip-on night sight is about to appear that may need all the top rail's real estate in front of the ACOG or whatever.  So there is some potential that because of the length required for a night vision unit, the front sight tower might need to fold down out of the way for objective end of the night sight to extend forward.
What is also important here is that the forend rail be as low as the upper receiver's original rail, so there is room out front later for a 60MM or so diameter objective lens.
ColdBlue sends...

The reason I ask is I have been offered a great deal on a used Bushmaster 14.5 inch with phantom M4 upper.  It includes front and rear GG&G flip up sights.  The rear sight will go and be replaced with an Arms 40 - personal preference.  If I were building one from scratch I would not put a flip up front sight on it.  I am a big proponent of KISS as well.  But it is there already.  I'm wondering if there are any advantages or disadvantages to this configuration.

I've also been offered a 14.5 inch with vortex M4 upper, this one a DPMS, that has standard gas block and A2 front sight.  Its cheaper, but doesn't have the front sight which I know adds value.  After I sell the GG&G rear sight, they will cost the same roughly and I would think that Bushy would hold value better than a DPMS.

Wanted to know if anybody felt that a flip up front was a liability in any way.

I am thinking about one too and would also like some input.  Has anyone experienced any problems with a PRI sight?

I bought one(GG&G) on my M4 upper because I wanted maximum utility.If I want a front sight it's there.If I don't it's not... very simple. I added Trijicon inserts front and rear.It's not an inexpensive option but in my opinion does add versatility/utility.Strictly a personal preference.

Versatility/Utility is the key!  If you are ever in a "bad" situation and your scope/dot optics fail, for what ever reason, you are screwed and the bad guy will kill you.  The BUIS are there as the name emplies BACK-UP! You may never be in this situation, but if it happens only once it's too late. FedEx can't deliver that fast!

I agree, and that is why I have a flip-up rear.  But what is the point of a flip-up front versus a standard iron sight?

Probably no point, so use the $$ to upgrade optics/equip.  In the example you gave, the fixed sight on a carbine would actually cause more distortion than on a full size rifle, but most quality scopes should "see around" the front sight anyway.  My rifle had no iron sights at all so I mounted both.

Well the upper I put a deposit on today, from a fellow board member, already has the GG&G front and rear sights installed.  The rear sight will get dumped and replaced with an Arms 40 - my preferred BUIS.  It will be a 14.5 M4, so a scope is not going to be used, but a red dot will - likely one of my Aimpoint Comp M2s.  I currently have a 16 inch M4 upper with a standard iron sight and do not find that it obscured my view for reflex type shooting.  I guess the sights do add the geewhiz factor and add to the value of the upper.

Bushmaster chrome lined 14.5 flat top phantom M4 upper with GG&G flip up front and rear sights with the side slinf adaptor on the front sight.  Used but excellent condition, $600.  Think that is a steal, especially since I will dump the GG&G rear for around $100 or whatever the value is on it.  I figure the upper will be around $500 when all is said and done.  I also love to buy from fellow board members... keeping the toys in the family.

If you want to be low profile while someone is trying to find you with NODs and you have tritium sights with the front and rear stowed they cant see those LED-like glowing balls of light when viewing with NODs.  

If you want to mount a Corsak laser to the top rail it will clear the front sight if folded since its not offset.

If you are easily annoyed by the blur of a front sight (like me) you will appreciate the folding front sight.

I find the HK style PRI front sight FAR easier to aim with than a standard front sight.

I had an Eotech mounted behind a standard front sight. It worked just fine, but I grew a wild hair, and decided to shave the gas block and put on one of BMs' folding front sights. I quickly found that I prefered the standard front sight with the Eotech.

I couldn't agree more.  I can't stand the iron sight in the way of my Eotech window.  Don't want to have to "get use to it" as some people have stated.  That's why I have the flip down front sight.  Still simple and I have it as a backup if batteries fail.  I can have it flipped up in a second so not an issue.