I've got an FM629, the rim brake version of the FM639.
The headset included was terrible, and the frame uses a non standard bearing depth for the top bearing, so you have to custom make a suitable spacer. Apart from that, the frame is great. However, it has a very slack seatpost (73 degrees) - be aware of that when considering the reach figures in the geometry chart. You might need to push your saddle fully forward to get it into the right spot if you don't normally run an offset seatpost (even though this is a straight seat post).
In terms of bike feel, the bike climbs beautifully. The steering is not very twitchy so it feels solid and tracks well even on the biggest out of the saddle efforts. But the flip side of this is that it isn't as sharp and precise on the descents, I've found it tends up understeering unless you force it into corners.