Well, I finally took the plunge. I have build 5235 (a private build) on my carPC now.
Speech is working great in that build. I'm really happy with that aspect... However, I'll have to keep booting XP until some problems with the audio stack for playback are fixed.
I can't get Windows Media Player to play any of my MP3s!
Nor can I get Frodo Player to work on that build either. I should probably check and see if Frodobaggins has a Beta copy of Vista to play with to make sure Frodo Player works.