Allow me to preface my answer with the fact that I'm disregarding any comparisons to Maemo 5, Linux and the Nokia N9. I'm also ignoring Ballmer and Elop. With that said...

Some people want Windows Phone 7, or WP7 hereon, because it offers true connectivity to MS Exchange, MS Sharepoint Server and have a corporate setup that utilizes those for scheduling, file sharing and whatnot. Portable MS Office without having to install an alternate OS just for OpenOffice or likewise can get a bit intense - but totally doable.

Some people want the games and Xbox Live integration. Having modern games that dole out achievements that are closely knit to your XBL account is seen as a good thing. I mean, I have a few games on my WP7 phone that I know I'll never see on older devices.

Don't want to use MS Exchange? IMAP, POP3 and Gmail are integrated quite well. All of my contacts (since I own an Android phone) are on my main Gmail account. I connect, sync the e-mail and contacts and I have all of the numbers that I tend to keep on my phone.

The UI... that's the part that I like and dislike. The UI is deceptively simple. Just a bunch of big *** squares with an oversized font and some scrolling. But honestly? It's that simplicity that lends itself to a faster experience going from the home screen to Twitter to posting an update. In fact, it's only simpler if you have a widget. But it's far from being customizable. Want control over how things look besides colors? Tough luck.

It's not Android... main reason for some folks... which surprises me. Android is a great OS. I actually like it quite well. But it's not exactly battery friendly - mind you, my Atrix goes all day, EASILY. In fact, my Atrix is on day 2 right now of usage, and I actually do call people and sync my 5 Gmail accounts as they happen without delay. It does seem to last all day, easily.

Mango adds "multi-tasking". Let me be blunt here... this is merely task switching. Some things work in the background, some things aren't optimized quite yet, some things just "freeze" and maintain their state. Without this task-switching, WP7 was not an OS I even liked outside of the fast UI.

Sure, folks will state that they do not like the lack of customization. Folks will say that they don't like the lack of openness. Some folks will state that they don't like Ballmer or Elop. Ignoring all of that, I will say that it is quite a limited experience. But as far as making phone calls, staying swift (UI), never really bogging down, integrating into my (unfortunately) corporate intranet/network... and entertaining me, it's a decent mobile OS. And by decent, I'd place it above prior editions of Android - Gingerbread won me over, I love Honeycomb despite its decidedly unbaked/unfinished OS feel. It's a consistent experience.

Things like the Me Tile, IE9 - which is LEAGUES above the pre-Mango WP7, music/Zune, Maps (Bing maps, I know.. but damn it's fast on lock) and for the apps that have it, the Live Tiles work great - I use Phonealytics for Google Analytics, I've never opened it since I set it up since I can see my targets, weekly site reads, and percentage (positive or negative) to my set up favorite sites.

The apps are growing. I don't have an FTP client, nor a SSH client, I can't admin my server from the phone... wait, those are negatives. Huge negatives. But there are some apps that are actually done quite well. And are constantly getting updated.

But it matters on what you want to do. If you're looking for a pocket computer, you're out of luck. If you're looking for a phone that won't stutter or hang when you're answering the phone and it has to rotate, you're in luck. And if you're looking for something that's a bit more finished and polished, well... I'd suggest using it first.

WP7 isn't a bad OS. It gets a really ****ed up rep around here, but it's easier to use than folks think. It's actually not bad when you're using it. And I went at WP7 thinking I was gonna hate it. Like seriously... I had no love for WP7. But now? I'm liking more than I dislike.

That's my take. Feel free to ask any questions.