I bought a USGI M7 bayonet and M10 scabbard from PKFirearms. It is a nice novelty item to have but is not very practical for civilians and is not a very versatile survival knife.
PK Firearms | Online Store
First of all, a typical civilian AR-15 with a 16" barrel and short carbine gas system will not allow proper mounting of the bayonet. The latch of the knife will engage the lug on the rifle, but the ring portion will fall short of engaging the flash hider. Instead the ring will engage the narrower barrel and be too loose.
For a Bayonet to fit you need a 20" rifle with rifle length gas system, 14.5" carbine with carbine gas system, or a 16" barrel and a midlength gas system (which is what I have).
Civilians (and soldiers) have been known to injure themselves with fixed bayonets. I know it is laughable, but strap a 12" double-edged knife on your emergency brake handle, and someday you may cut yourself.
As a survival knife the M7 lacks a waterproof compartment for matches or a saw edge. It is too bulky for carry when camping or hiking. A folding Buck knife would serve you better.
A mounted bayonet ruins the balance of the rifle, making it front heavy and ungainly.
It IS good for poking things. Of all my years in the outdoors, I maybe could have used one to subdue a 40 lbs stingray I caught bowfishing, but I probably would have put a nice hole in the boat too.
I reviewed my bayonet in the thread below (click "Fix Bayonets!"). I gave the standard warning I give whenever I mention bayonets. The thread got off track when people claimed they would never be stupid enough to injure themselves with a 12" double-edged knife fixed onto the muzzle end of their rifle.
http://www.xdtalk.com/forums/ar-talk/94817-fix-bayonets.html