I have both for a short while. Here's what I've found.
There's a reason why iMovie was designed for ipad2. It needs all the processing power it can get. Garage Band really demonstrates the differences between the 2 iPads.
On the original iPad Garage Band is a blast, but after building a few tracks it needs to "optimize" the tracks every time you change something or simply try to solo a track. It's actually rendering the tracks temporarily to audio to make up for not having enough ram or processing horsepower. And when it's rendering, it can drive you nuts while it slowly sweeps across your timeline.
With the iPad2 I find no rendering needed. You can mute, solo, add effects, etc. without the dreaded optimization pauses.
I've tested both iPads for web surfing using then same wifi network and separately on 3G. My original ipad is on ATT, and my new ipad2 is on Verizon. Both iPads have the same updated IOS. The iPad 2 on Verizon is a third to twice as fast loading the same web pages on wifi AND 3G. This was a real surprise me.
Overall, the iPad 2 is much more responsive, and the lighter slimmer form- factor is more noticeable than the specs would indicate. This is the ipad that the original should have been. A very worthwhile upgrade IMO.