I've got a set...they're ok, I got 'em at Midway at dealer price of $91. I would imagine they could be tough to get on without a handguard tool; you've got to press down really hard on the delta ring, line 'em up, then give 'em a whack to get 'em to seat right. A 3 handed job lol.
As far as function, they seem fairly sturdy, and they're tight, so they don't rattle around or move. The first samco stuff that goes on them can be loose, a few other folks have said that as well.
I think you might be better off with a bonafide rail system; check out the FIRSH that olympic sells...you might have to pay a smith to install them, but they're gonna be much sturdier in the long run. If you don't have a muzzle brake to worry about, you can install them yourself, really, if you have the tools.
If that isn't an option, the M44 are much better than just attaching a rial to the bottom of your existing handguards. I'd prefer to see the M44s a little cheaper than what they retail for, but for folks with no other alternative to the SIR, or who don't want to mess with a smith, they're a viable alternative.
They have a nice feel with the covers installed, too...obviously much thicker than the regular or even M4 handguards.
I've seen these for around $105 or so lately, so they're getting more attractive as the price comes down.