In the past I have owned three devices (all three made by HTC coincidentally) that ran Windows Mobile 5.0, 6.1, and 6.5, respectively. The common problem with these devices was lack of memory and seeming hardware issues. The first phone (PPC-6700) was my favorite of them, as it worked great until I dropped it (my bad) and then I started going through refurb hell with Sprint and upgraded. The second phone (Mogul) was better in all respects but a little dated in screen resolution, and the screen promptly developed large white spots that a quick Bing search revealed to be a common issue with overheating (phone design error) so the screen was unusable. I then was moved to a Touch Pro which worked fine but had a horrible form factor (thick on the depth and thin on the width) which made the screen very small and barely usable (form factor design flaw) along with horrible battery life (about 30-45 minutes of use per charge). After the issues with that phone I wrote off HTC devices and headed to the Sprint store, where a smooth talking sales guy suggested the HTC Hero with Android 1.6 (Cupcake). I figured I'd give it a shot based on the price ($69 w/contract) and was initially impressed. My honeymoon wore off quickly.
Android is an interesting beast and I've since used the factory upgrade to Android 2.1 and I'm no less pleased with it. Windows Mobile 5 was a better O/S with more features that I found lacking in Android. Android, following Apple's lead, is a siloed O/S where everything is driven by apps. This means information doesn't flow well between applications, and the base O/S has hardly no functionality at all. So for everything I want to do, I have to go online, find an app that does it (or not), and install it and hope that it works. Not my idea of good usability.
The biggest flaw is that there is no easy way to sync up outlook (that I've found) and keep contacts, mail, and tasks synced up. There is a download provided by google that runs on my PC to do this, but it's rickety and while it works, I still don't find the calendar app to be anything like what I had on Windows mobile.
I've been watching the news as Windows Phone 7 gets near and now I am very excited about a phone that has full Outlook sync, Windows Live, Bing maps, a good native browser, and the potential for apps if necessary. The only downside is that the phones are initially only available on GSM (not bad in the long run) but the models available by AT&T and T-Mobile haven't caught my eye like some I have seen overseas. Looks like I'll start watching ebay next week (Oct 20 release in Europe) to pick up one of the new phones unlocked. I'd even take Windows Mobile 6.5 on a good device over my current setup but looks like Microsoft might be showing up at the right time to make a decent dent in the smartphone market.