I work at an AT&T store and was given a Samsung Focus to demo for 4 months from Samsung. Let me give you some background info on myself, I have owned EVERY iphone from the original up to the 4, and never thought I would use anything else.
I really like the Samsung Focus/WP7 OS. It's easy to operate and really does bring something different to the mobile phone world (live tiles). It IS fast and responsive (except the marketplace...that lags like crazy...though there are 3rd party apps that basically bypass the native marketplace app). The notification system and email system are better than iOS IMO. The social networking integration is pretty awesome too (facebook profiles, profile pictures, and status updates show up under the people hub (contacts) and I can view all of this without having to go into the Facebook app itself)
That being said, it isn't perfect. It reminds me a lot of the the original iphone, with basic functions missing (copy/paste...multitasking...etc), and the marketplace (app store) is no where near as developed. If microsoft invests time and money into WP7 they might have a genuine product to compete with the iphone, if not, then right now as it stands it doesn't compare too well to the iphone.
I'm only about a week into my 4 month demo and haven't decided if I will keep the Focus, go back to IP4, or see what else is available at the time. Like pretty much everyone else has said though, this decision is USER PREFERENCE. Go to an AT&T store and play with each phone and see which one you like better!