I've used every WinMo platform from the original Windows CE landscape orientation devices, all the way up to Windows Mobile 6.5. Back in the day it was the best thing around, despite its frustrations. Nowadays, it isn't worth a look see unless you're already locked into it. I've tried the fixes and custom roms (XDA developers made it into my permanent book marks), I've tried the skins, and I've tried SPB mobile shell. And every new version (including the minor point releases) has always been the one to 'fix' its problems. Nothing can hide the fact that it's an incomplete, buggy platform, and as a regular linux and PC user for well over a decade I'm no stranger to buggy incomplete platforms. Performance seems sluggish and random, the interface is annoying to use even with a stylus, and almost impossible without, and the stock GUI is just ugly. Some of the skins, like the Sense UI are pretty, but only for the things included in the UI. You buy an open OS platform to be able to use other programs, but then you're jarringly back into the world of winmo.
Windows Mobile is the equivalent of if Microsoft had kept Windows 9x on the desktop, updated all the way into present day, without ever releasing Windows XP. Apple would have dominated the OS market had that happened, and that's what's happening in mobile. WinMo and Win9x were serviceable for their hardware back in the day, but Windows Mobile is long overdue for a Windows XP style upgrade. Windows Phone 7 could be that, but it looks like microsoft has turned it into a device platform, instead of a full featured OS.
As a decade long user of Windows Mobile/PocketPC products, I say good riddance. Blackberry, iphone, android, webos, symbian, anything is preferable to winmo.
(openness of the OS is a big priority to me, but even that's not enough to move winmo out of last place with me)
My personal rankings would be:
Android
WebOS
Iphone - I'd have to jail break it before I'd be happy with it
Blackberry - Not even in the same league as the other smart phones imo in terms of functionality, but it does phone and communication stuff really well, while the others are attempting to be multimedia/full computer devices.
WinMo - Maybe the newest point release fixes its problems, maybe all it needed was even more powerful hardware, but I'm done giving it chances.
Unranked:
Symbian - never used, but it seems like a similar like a similar workhorse to winmo, except nokia continually updates it and improves the development and driver framework, whereas all the development tools/frameworks microsoft is announcing for windows phone 7 SHOULD have been made available for the windows mobile platform.
(Personally, I'm on Nokia's N900, which uses its maemo platform. It has its problems, but I still prefer it to windows mobile.)
One last thing, anyone who does own a Windows Mobile device, I recommend checking out SPB mobile. It IS an awesome product, but a $30 add on shouldn't be necessary to make a phone user friendly.
Other awesome programs for Windows Mobile:
Coreplayer, Opera Web Browser (even paid for this back in the day, before it was free!), iGuidance (served my GPS needs for years until the charging dock for my car died)