I recently switch to OGG Vorbis too. I have no reason since I use Nero AAC double pass, but simply because I want to move toward free and opensource formats. I'm quite happy about it, seems the file sizes are smaller than AAC. And using FLAC for backup purpose.
I thought that using Q settings the bitrates will be variable, not constant?
There are two different encoder version, libVorbis and AoTuv, it seem people are prefering the latter. I also heard something about the upcoming version will have both of them combined, or something like that. Anyway, I get my latest encoders here: RareWares
It's probably easier if you use an GUI interface. I use foobar2000 which has its own converter, you just have to point to the location of the Vorbis encoder file when it asked. Base on foobar2000, Q4 = ~128kbps, Q5 = ~160kbps, Q6 = 192, Q7 = 224, Q8 = 256, Q9 =320, Q10 = 500
I think iRiver and Cowon devices support Vorbis, although I am not up to date with mp3 devices anymore. I use Rockbox
firmware for my players so it can support every audio formats.