If you know what you're doing and can figure out work-arounds for its limitations, iMovie is a fine little editor. People have done features with it.
However, Final Cut is so much easier to use (once you get past the initial learning curve) and so much more powerful that I would not recommend using iMovie unless you absolutely had no money to spend on Final Cut Express (or Pro).
It would probably take you at least twice as long to do a project with iMovie because it doesn't do much with audio, layers, filters and other stuff, and you'd have to figure out how to make it work.
imovie is a great starter and makes a bunch of things easier. Play around, try stuff out. If you find you are serious about video editing then upgrade.
I started out on imovie and learned a lot before becoming frustrated with it's limitations.
Someone please correct me if I am wrong, but one of the big upfront problems with iMovie is no support for 24p timelines. I assume with the FX1E you would be shooting 60i and deinterlacing, but you would still have the issue of how to edit. Of course you might have no interest in 24p, so this might not be a problem.