These forums aren't yet that active so sometimes it take a while to get a reply. I was actually working on a reply this morning but got interrupted several times.
I've used pretty much everything, but used Windows Mobile and Symbian (as a backup) before finally going Android with the Samsung Captivate, and then the Samsung Focus a couple months later. I'm not a business user, so this will lack Outlook and Exchange compatibility information.
Yes, I hated the loss of a unified inbox at first, but I've quickly gotten used to it. I've two main email accounts which occupy live tiles "above the fold" on my home screen. YMMV, but I realized that there was typically only one account that I really used throughout the day, and another that I'd use only a few times a day. My other accounts aren't even on my home screen as I rarely check them. This cuts out a lot of clutter and is less swiping I'd be doing than if there were a unified inbox, although I wish there were a quicker way to access them than going back to the home screen, swiping over to the list of apps/tiles, and finding those accounts. This is more of an illogical, OCD need I have to manually check all of my email accounts at least once every couple of days, despite the fact that I know I'd be alerted if there were new mail.
I love the new mail program. I love the fact that the tile displays how many new messages since I last looked at my inbox, not how many unread messages I have. I also love being able to swipe to those unread message or to flagged or urgent messages. I'm actually preferring this to having the swipe action switch accounts. Again, I live in one account through most of the day. Tapping on the left side of an email in your inbox lets you check it for deletion or moving within one tap, or if you tap on the ellipsis on the app bar menu you can mark them as read, unread, set/clear flags or mark as complete. You can also select multiple emails this way, very quickly and casually. I would like to see them allow you to tap the right side of the email to toggle flag status.
You can filter emails by hitting the search button and then typing a phrase to filter by. Exit filter mode by hitting back. If I'm not mistaken, I have a thousand emails in one of my inboxes. Not only is scrolling that list of emails buttery smooth, but it filters them instantly.
I'm not a huge fan of on-screen keyboards, but I've gotten used to the Windows Phone 7 keyboard. On my 4" Focus screen, the portrait keyboard feels about the same size as on the iPhone's 3.5" screen and the Tilt 2's 3.6" screen. In landscape, it's definitely smaller than the landscape keyboard on either because of the bars on the side for status and app buttons. Again, I've gotten used to it, but I can't type quite as loosely as with on other platforms, which brings me to my next point -- autocorrect.
It did take some time to get used to using autocorrect feature. Basically, words it's confident that you meant to type will be bolded in the autocorrect row and you can simply continue typing and it will automatically select those words. Words it's less confident about will simply appear in the autocorrect list but won't be bolded or added automatically as you type. I learned to just trust the suggestions, then go back, tap the words that need corrections and then tap the correct suggestion instead of correcting as I go. I think the Windows Phone 7 approach is faster for me.
The autocorrect dictionary feels like it was ripped from Word and as opposed to being a more fleshed out dictionary for texters like on most phones. There's a lot of names for people and places, trademarks, a few scientific and foreign words. It feels like sometimes there's too many words it contains, and this dilutes the accuracy of the autocompletion. I'm glad that it realizes I meant to type "vacillate", but "kanga" and "vassalage" (not recommendations for "vacillate", mind you) may bring a spark of joy to the few who use those words but I can't imagine anyone not expecting to have to add those words. Some fine tuning of the recommendations, like turning off certain types of words would be nice.
I also wrote
some thoughts on in response to another poster that you might want to read. They are also lengthy.
Edit: MS is aware of the fact that a unified inbox is an "often requested feature", so we may see it in an update. Also, I'd love a landscape QWERTY but I'm waiting to see how good the keyboard integration is post the next two updates before considering one.