iMovie is very limited -- about on the scale with Windows MovieMaker.(though it looks much cooler!)
Vegas is a full-featured editor that can run rings around iMovie.
But, if all you want is a no-brainer, single video track, drag-and-drop video editor, iMovie is just fine. And so is Windows MovieMaker. iMovie offers some very nice, semi-customizable DVD templates though.
I guess it depends on how serious you are about editing and how much you prefer to control and customize the results.
I would also say that one of the most limiting factors of iMovie is that it runs on a Mac. That severely restricts your choices of other software and peripherals.