What doesn't work is backlight applet (which is fixed in current opie already), sound settings are quite different now and little bit more complicated (gui currently doesn't work), buttons mostly doesn't work with opie and some other stuff like suspend.
So why even try it? From my personal experience, it works with more problematic cards. I have one card which sometimes works, sometimes reports a lot of IO errors. With vanilla it wors without problem.
WiFi
And the most interesting thing about vanilla. WiFi. I wish I could tell it works. Curently, we can say, that it partially works. You need to turn it on in PalmOS. And not only turn it on, you have to do something with it, like scanning for networks, so PalmOS initialize it. Then after booting to Linux, WiFi will be recognized as eth0. Scanning for networks doesn't work. Some other settings are broken as well. But some people reported that they managed to get AdHoc network working, so you can try, but don't expect so much. Once I'll have some step by step howto, I'll post it here ;-)
Just one final note - WiFi related thread on Hacking and Development.
