My current cellphone has been trending downhill for the past few months. It's an HTC Touch Pro device with WM 6.1, and I hacked it to run WM6.5 that I downloaded at xda-developers. Those great hobbyists do a wonderful job at packaging roms which unlock the full value of phones. I've tried a few different roms but the "Energy rom" line was the most stable and best one I've seen.
Now I'm sure you're thinking "the phone worked fine until you hacked it". This is the opposite, the phone out of the box from Sprint was buggy, laggy, and was a fail at basic telephony. Not to mention the habit of draining the battery before lunchtime. By researching and installing a custom rom I was able to get another 6 months of use from the device, else I'm certain it would have ended smashed into a million pieces on a sidewalk somewhere.
This leads us to today - WM is coming out with WM 7 next year. This is supposed to be an awesome system. WM 6.5 excels where Apple fails. It's open development, lots of good resources but on the flip side a lot of the usage is wonky and seems 2003-ish. I cannot wait for a better system in a year from now when I need a smartphone today.
I purchased a HTC Hero last week running Android. Android is made by Google and is quickly gaining market share among smart devices. It didn't have a dedicated keyboard so I thought I would be bothered by the virtual keyboard, but so far it's not too bad. The phone works well and it seems that the flow and use of the software makes sense. It's much more user intuitive than the Microsoft platform as it exists today, but I'll wait for the newness to wear off before I comment on the overall package. If Microsoft is destined to have a presence in the mobile market, WM 7 must be a compelling "wow".