I've been running my piston system for a few months now & have had no problems. I bought it from Primary Weapon Systems (www.primaryweapons.com) and mounted it on a LMT Defender lower.
The piston system is a bit heavier than a comarible DI system, but not enough that it is bothersome. The rails are much more stout than other mfgs so I presume the additional weight comes from there.

The recoil is much LESS than a DI system. The action runs cooler after sustained fire, although the barrel does get hot still. Without the normal cycling feel from the bolt and buffer, I felt as if I could run the gun faster, similar to the difference between a DAO gun and a 1911.
Cleaning time is reduced greatly as the majority of the gas hits the piston and nothing else. The upper receiver is clean and my hours with dental picks and q-tips are gone!

As soon as I got the upper I ran 700rds through it without any oil, lube, or grease before it started to stick and not feed completely. Adding some TW-25B grease solved the problem immediately.

My only complaint is that parts can be difficult to find at a match or if used overseas, however prudent planning can solve this.

I would recommend a piston system (not a conversion kit) for anyone who is slightly considering it.