I have a Powermate, I love this thing. My iBook would not be the same with out it. It makes scrolling so much easier in Safari, Word, Excel, Mail etc. I haven't used it with iMovie yet, but I heard it works really well.
Yeah I've picked one up at my local Apple store. Got the brushed alluminum version although the black one would have complimented my G4 quite nicely. It's pretty cool although I use it primarily to control the system volume and haven't used it navigate iMovie and FCP3. There's a blue LED light located at the bottom and in the System Prefrences you can set it to pulse which adds a pretty cool effect. Since they've come down in price, not a bad investment. Oh, and by depressing the top mutes the volume. Great when that phone rings and you've got some tunes rattling the ceiling plaster.