I wrote this three years ago, and since then, I continue to say my SR-556 is my favorite rifle in this platform...
[WAYNO]A little background, first. One of my MOS's in the Army was small arms repair. That means I repaired every gun and mortar the Army owned, from bore diameter 4.2" and smaller. By a huge margin, I repaired more M-16's than any other firearm. The gas system was very fragile, and I learned to not trust AR's very much. Still don't.
After a lot of years, I recently bought a new Bushmaster M-4. Gave me such fits, right out of the box that Bushmaster had me return the gun to them. They replaced the barrel, and it's been perfect ever since. This gun shoots better than any AR I've ever shot or owned, previously.
Then, along comes along the Ruger SR556. I bought one, and I've gotta say, as much as I've learned to like my Bushmaster, I like the piston-Ruger even more. It runs perfectly, and is unbelievably accurate. The action remains clean, and the gun runs so much cooler. But, this gun is hefty. Maybe hefty is part of why this gun shoots so well.
So, is there an absolute need for a piston-AR? Maybe not. But I sure do like mine.
So, the OP's question is a little two-edged.
Is the piston a worthwhile improvement? Yes.
Is it worth the huge increase in cost? Probably not.
Regarding price...The cost of the Ruger has come down quite a bit. I paid $1400 for mine, just a few months ago, and had I waited, I've seen them for $1300. Maybe the price will continue to come down.
WAYNO.[WAYNO]