Piston Rifles are not new. They have been around for years. The only problem I have with buying piston in an AR15 is that the systems are proprietary. If something breaks in the piston system you are going to be down till you get the part in. Plus there is no clear cut advantage for it over owning a DI AR15. You can build an AR10 in 7.62 but the ammo costs are higher.
This should be your biggest concern if your building one. If you get a kit from some fly-by-night company and they go under your screwed if you ever need a part.
Now back to the piston v. DI debate. I have 7 AR's and only one is a piston model. Its an Adcor B.E.A.R.
Pro's: Very nice rifle, shoots great, bolt seem's to never get dirty.
Con's: A little bit heavier, louder, cleaning under the handguard is a pain, expensive.