On the whole, I'd say "No, stick with iMovie 2"
I've an iBook 500 with 384 megs of RAM, and even with NOTHING else running on 10.2.3-->10.2.5, performance is sloppy and sluggish (pretty objective, I apologize, but it IS rather difficult to get benchmarks on GUI functions), and on the whole I'm displeased.
I was hoping that the (relatively) poor performance of iMovie 2 under 10.0.1-->10.2 would be something fixed with 3, but it looks like I'll be reverting BACK to iMovie 2 sometime soon.
Without a Superdrive to allow me to take advantage of the iDVD-integration features, iMovie 3 just isn't really worth it on our (my) machines, in my opinion.