My wife has a Pre and I have the 3GS. Here is a quick review of my experiences.
The gestures on the Pre take a little while to get used to coming from the iPhone. After you get the hang of it though, you're blazing through the screens quickly. I found the touch response of the screen very good. It isn't 100% as snappy as the iPhone but it is miles better than anything else that I've tried the touch screen on.
I do like the multitasking on the Pre. I like that I can have several apps running at once and switch between them quickly. After having used it quite a bit, it does make Apple's version of multitasking (i.e. using the phone or ipod) seem antiquated. It just felt clunky coming back to the iPhone and having to close an app, open the next app, do my work, close that app, and reopen my first app. On the Pre, I can just bring up the new app and either close it when I'm done, or just leave it there. I wish that Apple would think about multitasking more.
The camera is good. I really like that it pushes the rendering and saving of the picture to the background. That makes it easy to take pictures quickly. Just snap, snap, snap away. On my iPhone 1.0, it took FOREVER to take a lot of pictures. On the 3GS, it isn't as bad. Browsing pictures on the Pre does take a while. Its rendering is laggy and I got found myself getting tired of waiting. My advice, snap a lot of pictures on the Pre and pray that one turns out.
My wife doesn't have a Gmail account, so we don't take advantage of the push features like we could. However, I don't like that you have to pay for a Mobile Me account just to get the same features as the Google ones on the Pre (which are free). Yeah the "find my iphone" feature is one that the Pre doesn't have. But, everytime I'm indoors with my iPhone and turn on the GPS, it thinks that I'm anywhere from 10 miles to 30 miles away from where I'm actually at. Point being, don't lose your phone indoors.
Yeah, Apple can laugh at the number of apps that the pre has at launch, but how many did Apple have when 1.0 came out? My wife just wants the basics anyway and didn't need her choice of 50,000 apps. She uses it as a phone / communications device.
I was able to quickly add all of the music and movies to the Pre through iTunes. The media player on the Pre is, in my opinion, just about as good as on the iPhone. There's no coverflow on the pre, but I never use it on my iPhone unless I'm trying to show off. I don't need to make playlists on the fly, so a lot of the advanced media player features of the iPhone are lost on me.
All in all, for me, I still like my 3GS. If there were no iPhone, the Pre would be a superstar. It is a good second choice if you don't want to get an iPhone or just want a cheaper data plan.
Just my $0.02.