I own both, and I'd say for hobbyists (like me) who don't do super lengthy or super sophisticated video, that for editing on its own (not other things that need to be done in Motion, etc.) there isn't a huge difference. You are limited to a few codecs when editing in FCE (but the codecs you can use are pretty good ones) but on the surface, for hobbyists, FCE seems very close to FCP. (This has been a source of amazement for many software reviewers, who can't believe that Apple gives so much away in this consumer product.)
If you want or need the other apps (which are greatI'm just starting to learn Motion and it's wonderful) or if you want to go pro, then you would definitely need Final Cut Studio. But if you're a hobbyist, or someone who is interested in using Final Cut but isn't sure about investing all that money yet, then you can get a pretty clear idea of what Final Cut Pro is about by using Final Cut Express. The interface differences (at first glance) between the two are not glaringly obvious, so if you get used to using FCE and decide to jump up to FCP, you won't be re-learning everything.