I have the HTC HD7 running Mango. Made the switch to ICS 4.0.2. Overall, I like ICS more than Mango.
Multitasking in Mango suck major donkey ass. To close an application, say IE, you have to press the god damn back button a billion times, if you visit a lot of websites; whereas, ICS, you just swipe and it is dead.
When accepting a phone call, WP7 will only vibrate once and then continue to ring; whereas, ICS and iphone will ring and vibrate until you pick up; which is great when you are in a loud environment such as a jazz club or lounge or a dance club.
I love how WP7 made use of Bing Vision and I feel this missing feature the most when using ICS. With Bing Vision, I would fire up the Search and have it read qr tags, ms tags, bar codes, cover of a book, grocery products, etc ... for additional information. This is something that iphone and ICS doesn't have.
GPS on WP7 suck ass as it wants to get you in an accident/kill you more than get you to your destination. Instead of continous voice guidances, it give you a damn tone. And to get the next direction, you have to touch the screen, this would mean you have to take your eyes off the road.
WP7 does have a nice feature that is also sorely missed is the Local/Scout feature. It find the latest happenings and events around you. In addition to this is Xbox Live access on the phone, not that I use it a lot but it allow you to quickly add friends and check msg that is stored on XBL; except that it can't playback voice messages stored on XBL

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MSOffice suite on WP7 is nice except that I can't use sharepoint unless I'm within corpnet, which is pretty pointless since I'm at work; therefore, I will have my desktop/laptop w/ me in meetings and such, so what is the point of pulling up sharepoint via the phone? I don't think this is MS problem, the network/system admin would have to allow access to sharepoint portal outside of corpnet.
I also love the WP7 intergration into facebook and twitter. The part I dont' like is that it pull down every god damn contact in fb & twitter and inject it into my phone when I only want their updates. Also, whenever I add a new friend on Live Messenger, it create a contact for that person in my address book and if I delete that address book, it delete the contact in Live Messenger, which I loath. Just because I add them to Live Messenger, doesn't mean I want a contact created automatically.