Here is a good article of why NOT to buy a piston AR: Why NOT a Piston-Operated AR-15?. Granted it is another man's opinion, but it he does make good points. The author is the gentleman who developed the M4 chart.
I know Rob S. is considered one of the poobahs of the AR internet, but a lot of his issues have been dubunked or don't necessarily apply to a range commando. I've seen the article before and I'm not sure how old it is. Carrier tilt seems to have been taken care of with the quality kits (LWRC, LMT with some minor tilt initially, and Adams Arms with it's BC and "skis"). Paul Santos and "Quib" (two other poobahs of the 'net) did some articles online at WeaponEvolution, one w/ a LMT conversion and the other with a Spike's Piston (w/ Adams Arms) and they both loved them and didn't find them any heavier (other than an ounce or two). Accuracy was on par with a DI gun, as well, which is another often-touted "bad" thing about Piston guns.
Rob's other argument is that it's polishing a turd and if the parts aren't good to begin with, it's not helping. Well, no duh. That's true no matter what you add. I've always been one to spend more money on initial quality to make something last than buy the cheaper one which may have its issues sooner (I'm talking any product, not guns). So if I build a piston gun, it's going to start w/ a decent setup and then have a well-reviewed piston, rather than buy a cheapo Gun MegaMart M4 and then add the cheapest piston setup.
As it happens, I'm thinking about going down this route with a SBR and an Adams Arms kit. I'm not sold on the idea yet, but I'm getting there. I have plenty of time to figure it out, though, once I send off paperwork to the ATF.