For what it's worth, I have very little experience with quad rails or freefloat tubes.  That being said, I got to play with a set of the new Armalite quad rails today.  

It appears that they are similar to KAC railed handgaurds that are not freefloating.  They are machined aluminum and simply replace the existing handgaurds.  They are very solid and have several screws on each side to hold them together.  The machining work and finish were both great, and the few accessories that we tried on them fit perfectly.  Although I still don't know that I would want to mount on optic to these.

Sorry I can't give more info, today's the first I've had a chance to look at them.  It looks like the price is going to be considerably less than the similar KAC product for those people that want rails, but aren't concerned with freefloating.
Scott