I bought a set of FG-51's and RM cavity backs earlier this spring.
I agree with Billybogey. I'm hitting my FG's much better than I did my game improvement irons. I haven't even played with the RM's yet because I've been so happy with the FG's. Just yesterday I hit my 4 iron 180 dead straight to about 10 feet short of the hole. I don't think I hit my game improvement 4 that straight or far ever. Mostly, I think, because it was so hard to align the things with the target properly with any sort of trust. My eye just hated the offset. I look at most Callaways and wonder what part of the garden it's used in.
With the fact that you can get a used set so cheap on Ebay, I would reccomend to anyone with a decently grooved swing to pick up a set just to practice with if nothing else. Then maybe they would get over the intimidation factor and quit believing the hype that the club companies play up.