I had a FMF pipe it was ok. I switched to a lrd and it was one of the best moves i did. Everybody loves the esr trx5 pipe from what i hear on here. If I find one used I am going to pick it up and give it try to see what everyone is talking about. But back on topic the fmf is a ok pipe not the best but not the worst IMO there are better pipes out there.
On the old forum, there was a guy that went from an FMF to and ESR trx6 and an FTZ on the dyno. The FTZ and ESR made about 6 or 8 more horsepower. The TRX6 is an mx pipe and the FTZ is a TT/drag pipe so depending on your type of riding I'd say any ESR, FTZ, or many others is better than FMF.
I have a TRX5 on my 350 but dont have enough seat time to give a fair review of it.
I always thought a pipe was a pipe till I went from my pro circuit(which I sold my FMF to buy) to a Sparks TT on my 275 and it made a world of difference. Not only did it have more power all around, it was smoother, easier and more fun to ride. Its funny, just by looking at them, the shape was barely different between the two, but you sure could tell a difference riding it.