WP7 is VERY colorful. It's customizable in the fact that pictures are used as wallpapers of everything. Each live tile is unique. My "People" (Contacts) live tile would NEVER look like your "People" live tile. Therefore, without patches or extra add-ons, its already very "customizable". As stated above, its VERY fast, VERY responsive. I've yet to see a list have even a half second delay between when I slide my finger and when the list moves. It's instant and a huge refreshing change from webOS (sadly enough).
The Dell Venue Pro is the ONLY respectable portrait slider phone available today. It looks really good but I have yet to feel it. Grab a nice cheap 8 GB one and drop a 32 GB microSD card in it (will save you over $60). I'm very excited for WP7, even though it feels a lot like webOS in that it doesn't have TOO much support (way more than webOS already, though). Mango is fantastic. It's a huge upgrade from the NoDo (which is, I think, the previous version).
My biggest complaint though, is the fact that its impossibly difficult to hack. Custom ROMs are possible and every day there's an announcement about it, but it's very limited (side loading apps is next to impossible). Windows also is holding the reigns very tight on developers. Not too much access, you have to officially proclaim your device as a developer device just to beta test your apps on it (you can only have 3). etc. etc.
Its a change from webOS, and the future looks a lot like webOS (good potential, nice refreshing UI, but will it pick up enough to stay alive?).
Thats my two cents on WP7. You can read my blog posts about it for a bit of the same info I just condensed here: A Temporary(?) Dabble in Windows Phone 7 | Sirata Studios