Well my opinion is totally biased, but I personally use
RemoteAmp:
I definitely agree with flessan though, if you were to use this in your car it would need to be mounted and I'd only use the hardware button functions. Trying to tap something with the stylus while driving could be a bit tricky.
Update: Missed your second question. You can also use VNC with a Pocket PC to remote control all desktop functions, or use Terminal Services if you have Win 2K Server or XP Pro. My personal preference is to go with Terminal Services but VNC definitely gets the job done.
As for how well all this stuff works, it actually works quite well. The things you can do with a Pocket PC anymore continues to amaze me.