iMovie and FCE both have their places. I use both for different situations, and I am currently working on a project that I expect to use both on.
That said, there is a curve with FCE, but it is very manageable, depending on what you want to do. I took the CD with the manual to work, printed all 700+ pages and had the receptionist bind it for me. I use it more as a reference when I am trying to do something that I don't already know how to do.
iMovie is extremely capable and if you check out Erica Sadun's Book, "iMovie 3, Tips, Tricks, and Solutions" (or something that) you'll find that iMovie is WAY more powerful than most people know. It shows you how to do compositing and all sorts of stuff most people would think to try in FCE/P. Most of the techniques require QuickTime Pro, but at $30, it's more than worth it.
I only purchased FCE because I knew this person that had gotten a free copy through some promotion at an Apple Store. He didn't want it, hadn't opened it and only asked for $90. That was worth it to me.
Anyway, I use both, and will continue to do so...but I would recommend getting Sadun's iMovie book, a copy of QTPro and learning the really cool stuff you can do with iMovie. The biggest advantage to iMovie being it's extrememly fast work flow (at least for me). I can get a lot more done in iMovie in a given period of time, but I don't have as much control. FCE comes in when I need to do something iMovie/QTPro can't handle easily enough.
Hope that didn't make your decision harder.
Good luck,
Eddie