I've used webOS, Android, iOS and now WP7. Here is my opinion of WP7 so far:
Pros:
1) Absolutely agree, the UI is very fast, buttery smooth, and responsive. It also has a very cohesive, consistent feel to it that is very lacking in Android. WP7 almost hits the same level of smoothness that iOS does -- that's a major accomplishment for a 1.0 OS and Microsoft deserves high praise for it.
2) The lockscreen shows useful glanceable information. Unless you jailbreak an iPhone, nothing else comes remotely close.
3) Keyboards. I'm using a Quantum, and the physical keyboard is very roomy and responsive. The virtual keyboard in WP7 is outstanding, even slightly better than the iPhone's.
4) Live tiles are a great concept. Only Android gives you something similar with widgets, but WP7's approach is superior IMO. All I really need to know _right now_ is shown: when is my next appointment, are there any new emails or messages, and did I miss any phone calls. Android widgets are usually poorly designed for getting information quickly. webOS has no widgets at all, and you have to jailbreak to get similar functionality on iOS.
5) The WP7 calendar is the best I've used on any mobile OS. It's fast, efficient, doesn't bombard you with extraneous details, has a live tile, and has lockscreen integration. If you live and die by your calendar, you'll completely love WP7.
6) Most of the "must have" apps are already available for WP7. Netflix, Facebook, eBay, Kindle, TuneIn Radio, Twitter, good quality game ports, etc. Unless your app needs are specialized (IT support, for example), the WP7 marketplace compares pretty well to the other mobile OS app stores. (What's with all the "how to tie a tie" apps, though?)
7) The text insertion cursor in WP7 is brilliant. It works intuitively and accurately. I hate the iOS "magnifying glass" method, which doesn't always work very well near the edges of the screen. It's hard to be accurate with webOS and Android. MS nailed this feature. This is the first mobile OS where the insertion cursor is actually usable. It's often faster in all the others to just backspace over mistakes and start over.
Cons:
1) The notification system is good, although webOS is still the benchmark with Android behind that. WP7 notifications don't appear consistently though, especially when using an app. The iOS notification method is too disruptive and primitive, but at least I know I'm going to be interrupted reliably. MS needs to do some tweaking here to make notifications truly reliable.
2) The browser is slow compared to Mobile Safari and webOS. I've seen comparison speed tests between WP7 and iOS, but I can't come close to duplicating those results.
3) Downloading pictures in email is blocked by default, with no obvious setting to change this behavior. You have to manually download embedded images every time; these are not cached as far as I can tell.
4) No tasking integration with Exchange. To be fair, only webOS does this without 3rd party apps, and it isn't very robust anyway. This should be very easy for MS to do and it's a golden opportunity for further meaningful differentiation between WP7 and iOS/Android.
5) No copy & paste. Yes, it's coming and not everyone cares about it, but there really isn't a good excuse for WP7 to be missing this feature in the first place.
6) Minor bugs and hiccups occur from time to time, which is normal for a 1.0 OS. WP7 runs rings around Android in reliability and stability, though. It's almost equal to webOS already. iOS is still the best in this category at this point in time.
7) The camera hardware specified by MS isn't particularly advanced, and yet the camera app and picture quality is inferior to that found in other mobile OSes with similar sensors. I do really like the dedicated camera button. However, the button doesn't seem to consistently launch the camera app on my Quantum. The image processing software needs major improvement to compete with iOS.
8) Can't connect to hidden SSIDs. Wifi connections aren't very quick or robust. Android, iOS and webOS are all better and faster at making and holding wifi connections.
Overall my personal ranking of these OSes would look like this:
#1 - jailbroken iOS (Custom ROMs and overclocking webOS don't make nearly as much a fundamental difference as jailbreaking iOS does.)
#2 - WP7
#3 - iOS
#4 - webOS
#5 - Android
HTH. I think you'll be very pleased with WP7. If you want a mobile OS paradigm that is fundamentally different than Android/iOS/BB, you'll really love the experience. webOS is slightly more elegant, but WP7 definitely beats webOS when it comes to meaningful "right now" productivity.