Though Final Cut Pro is superior to iMovie by design, it is exponentially more complex. I have a belief in using the right tool for the right job, and in many cases iMovie fills the needs of a project. There is simply no faster way to slap something together than iMovie. But, if you want a highly stylized and time sensitive product, than you need to use a tool that is capable of delivering that... iMovie is not that tool. The question is this: Do you have long term video production aspirations, or are you someone that wants to dabble? If you do have long term goals than get FCP, not only because it's the right tool, but also because it shares similarities to realtime editing systems like avid. If your just dabbling, and I think that's the best part of computing, I'd say if you want to put together some longer format stuff, and really get into controlling every aspect of your video than FCP is for you too. However, if you don't care about precise time, longer format, or greater user control there is nothing wrong with iMovie.
Dang sometimes you start typing and you can't stop,