There are things, like the gestures, that only work in iPad 2s and not originals. And there are people that say the update kind of slowed down their iPad 1s.. But I have a 2 so I can't testify to that. I do think the things like the notification center and the split keyboard (which I'm using right now) are worth it! I also think a lot of the update problems were caused by the server overload.
I would hear from other people with iPad 1s, but I personally think it's worth the upgrade, especially for your iPhone!