I've just started using one with my HVX200. Initially I had some issues with blocking and subsequently Focus put a firmware update out which seems to have solved the issue for me, although others report the issue even after the update.
As FCP ( and most other NLE's) has no support for 720p 25/50 I have been mainly using it to capture 1080 25p and it has performed very well. I have done some 720p tests using the HD Log 720p 25 workaround and again this was handled fine by the FS100. Not yet done any DV25 or DV50 records, where you can specify more recording options.
The menu layout is quite deep and a lot of options are offered, but once you have got your head around what you need for each setup it becomes quite a simple process.
Overall it seems like a good product on release (apart from the afore mentioned firmware upgrade) and should hopefully be kept updated via the firmware as and when any other issues arise.
Mostly happy FS100 user,
Mark