iMovie - which version?
are you using? As others have said, the latest has morphed into something different, with less editing capability.
If you ARE using iMovie 6 - what limitations are you seeing? It's been a while, but I thought it was pretty good. IIRC, the limitation that I had was trying to 'overlay' one video on top of another (probably not the correct video term). The video eq of 'mixing' two audio tracks. I think that required some more advanced tools. Some of the pay-for plug ins really add to the tool box, maybe even provide this layering.
-KCMW