I have a Fujitsu-Siemens T830, WM5, GPS onboard and PocketFMS. Works pretty well although the screen is a little too small on the 830 for anything serious (240x240) - if my HTC TyTN or 9100 had GPS on board, it would be much better! Some of the non-phone Fuji PocketLoox PDA's with integral GPS look good too - might try that as another solution and maybe give MemoryMap a go.
No problems with stability on any WM5 device I've used - provided you don't install too many (or badly written) Today plug-ins. The Resco today plug in is particularly flakey I've found.
I use it stuck to the panel/yoke as a convenient way to record my route and use the Garmin 250XL in the panel for normal nav, however the PDA if the screen was bigger would knock the socks off the 250's monocrome (and tiny) screen.