I use x51V and so far so good. I bought it instead of a PDA phone because for the price difference, I can buy a very nice phone separatly. I prefer to have a separate phone because sometime I need to carry just the phone and not the PPC. Also standalone PPC has bigger screen. For example if a x51v cost $2,900 and a Sony Errisson W810i cost $2,600 and the total is only $5,500 which is the same as a PDA phone. But you get a bigger screen capable of running 800x600 true VGA mode and also faster processor.
I too had a lot of device hang problems at the begining but after I move some of the program (such as Wisbar Advance) to the device instead of the SD card I don't have a single hang for weeks.
For Chinese system, I am using CE-Star Ver 2.8r1 which is for WM5 and it is very stable. In terms of GSM card, I prefer to connect my x51v to a BT phone instead which give me more flexibility.
Just my experience.