I can only speak for the Mogul...
RAM is way too little. Open up GPS + WMP and your RAM is pretty much shot. You'll experience pauses in the music every now and then. Open up something else like Opera and it'll have a good chance of crashing. Stop all these programs and your RAM doesn't get freed because WM6 memory management is horrible.
Activesync and Sync Center for Vista are complete and utter POS. Sometimes they will make it so that you can't put your phone on standby. They will wake up your phone automatically each time you do so due to a bug.
When syncing, the Mogul essentially becomes a brick. Nothing responds. If you have a broken Activesync that likes to "check for changes" every 5 minutes, the phone becomes useless hooked up to the computer.
WM6 has a special option just for hooking up an external bluetooth GPS. This doesn't work at all. It can hook up to an external GPS though, just not through Microsoft's sh*tty implementation.
Battery life can be stupid and horrible. You put the phone on standby and come back 2 hours later to a fully depleted battery, with no idea WHY it's now so low. And then there are times when it doesn't get depleted.
The Microsoft Office Mobile products are laughable at best.
With this said, the Mogul can do a lot of cool things and be a lot of things to the user. GPS, note taker, web browser, phone, planner, etc. The only thing that's really wrong with it is the lack of memory and Microsoft, uh, everything.