I have several FMA Rx's in the "extreme" line. They work great in the air. But they have proven to be a bit fragile. The FMA cards and/or solder joints don't seem to hold up to combat abuse, at least mine havn't. I will say their customer service and repair center are great, and fixed RX's come back quickly when I send them the broken ones. I have 3 broken FMA's on the shelf right now, waiting for me to get a box for them. And I have 4 more that are currently in service.

I do find the caseless ones break easier, so I've been having FMA convert my Extreme 5's to Extreme 4's. It's the same RX, they just clip off the 5th set of pins and put a case on it. The case seems to support the board and reduce damage. I also only use the FMA RX's in full-fused models, no profiles or planes with exposed bits. Combined with some extra foam padding, they've been doing well enough in my SSC planes.

It also helps that SSC planes don't hit as hard as the B or 2610 class planes.

I haven't tried an M5 or the bigger FMA rx's so I can't tell you if they work better or not.

Oh, the FMA RX's I have also have really thin wire for the antenna. It's a lot more prone to breakage, and harder to solder back together than the Hitec antennas. But the FMA's actually work amazingly well with a shortened antenna. If you look on their website, they explain why. I have a 2 yr old FMA in an SSC plane which has an antenna that was chopped in about half in a mid-air years ago. It keeps working (and range checking just fine), so I keep flying it.