I have an HTC Mogul running Windows Mobile 5.
I'm quite pleased with this phone. I was basically in the market to get the most computer-like handset Sprint had available. I use it for internet and applications WAY more than I do actual calling, so I have unlimited data transfer, but use less than 200 minutes per month.
Anyway... it hasn't crashed once since I got it a couple of months ago. Frozen a few times, but I'm completely to blame... 2-3 tabs going in Opera and multiple apps running at once will do that.
I have a sneaking suspicion it might not be as sturdy as a more basic handset, but I have a pretty serviceable Samsung A900 to fall back on if this one bitches out on me. That phone crashed ALL THE TIME on the net, but worked very well for calls, messaging, texting, etc.