I think he's talking about the AR-15 conversion kits that have become popular, not simply the DI vs piston design.
IMO, the AR-15/M16 design was never meant to be a piston operated weapon, and so there are drawbacks to installing a kit. The biggest one is the carrier tilt which can lead to buffer tube wear. The piston rod/shaft impacts the carrier off center (non-coaxial load) causing the front of the carrier to tilt up and the rear to tilt down. Although there is an anti-tilt carrier design, it cannot eliminate the force that causes the tilting motion in the first place. So the force is still present. The anit-tilt carrier may minimize the buffer tube wear, but it will eventually wear. The tilting carrier can also wear on the upper receiver prematurely.
If you really hate to clean your rifle and don't mind changing out the buffer tube now and then, it's probably not a big deal. Personally, I like cleaning my gear and don't want to add extra parts to the rifle that can fail in the heat of use. It's a small probability, but I don't know if I think the advantages of a piston kit is enough when a DI rifle will work for thousands of rounds without cleaning, just oil periodically.
The two kits that seem to be big right now are the CMMG kit and the Adams Arms kit. The Adams Arms kit started out close to $500 and uses your carrier while they provide a key that you have to install. They are currently having a sale for the kit at about $250.
Adam Arms Kit Discussion:
http://www.ar15.com/forums/topic.html?b=3&f=124&t=374524
The CMMG kit comes with an anti-tilt carrier and key machined as a single piece. The latest version has a steel gas/piston block that appears to be a better design. I'm not certain it has been installed on a rifle length gas system, but CMMG can verify. It retails for $299, but AIM has it discounted $10 or so.
CMMG Kit Discussed:
http://www.ar15.com/forums/topic.html?b=3&f=124&t=452885
There are fewer choices of rails or hand guards due to the space requirement of the piston kit, but if you like the basic M4-esque hand guard the CMMG comes with it. S&W uses the Adams Arms kit on their MP15 rifles. Spike also uses the Adams Arms. CMMG also sells a complete piston upper for about $675 with a 16" 1/9 barrel. So you could have the best of both worlds if you keep the DI upper.
I may one day play around with a piston kit (the CMMG looks good to me), but for now I don't think it's necessary.