I have an Audiovox SMT5600 smartphone, running Windows Mobile 2003.
I have it for a bunch of reasons:
-- For most daily uses, it's more convenient than carrying around a phone and my PocketPC
-- I like the Windows Mobile platform, especially when looking at development opportunities
-- The battery "clips" on my LG VX4400 were broken, which required me to secure the battery with a zip-tie around the phone

-- I wanted to drop Verizon service, as I was having a lot of signal problems after moving to Arkansas
-- I wanted to get Cingular due to the great coverage out here (and in Alabama), and we have Cingular Premiere available to us through work
I've had my SMT5600 for about 6 months now, and have had absolutely no problems out of it at all. The coverage is great through Cingular, calls are much better quality than I got on Verizon, and I have had only a couple of dropped calls. It provides enough functionality that I can ditch my PocketPC 90% of the time, and still be able to do all the things that I need.