The new house has a sprinkler system. Ended up having to replace 3 heads because they'd been chewed up by edgers/mowers, so I decided to do something about that. I don't like the concrete rings...they're bulky, wide, hard to cut into partial rings, you can't adjust their depth (they sit on the ground) so if the sprinkler head is taller than them, then they don't work, etc.
I ended up buying a 10ft piece of 4" PVC "gravity sewer pipe" and using a miter saw to cut it into ~5-6" long pieces. It's schedule 10, so pretty thin, which makes it easier to hammer into the ground to the depth needed for each sprinkler. I did this with a piece of wood on top of them and a 4lb sledge hammer. I even cut some into partial rings for places where a full ring wouldn't fit, like when a sprinkler head was up against something concrete, e.g. driveway. Bonus: the pipe is green, so it doesn't look bad in the yard.
I think they look better than concrete rings. They're cheaper, too. Win-win-win