Good or Bad?

just some minor quality difference. Not a big deal for me thou. I guess it also depends the type of music. I notice if i have real heavy-bass music, they tend to be louder/with more bass at 320.

Along with kamj12, I heard a difference in bass response as well. All my stuff is in 192 kbps CBR, but for testing purposes, I encoded 3 of them into 256 and 320. Hard to test because the pause between switching tracks, but playing thru my home system, I heard more articulate, pounding bass (primary test CD 'A Tribe Called Quest - Midnight Marauders') from the 320 than the 256 than to the 192, all compared to playing the CD tracks. I don't know about that research I found out that 256 is better than 320, but I beg to differ. Never heard AAC tracks, but have tested VBR and its not much better than CBR so don't worry yourself with that.

Conclusion: 192 bitrate is good for general purposes (lowest I'd ever go), 256 is getting a little better high end, bass just a step up, and 320 had the high end shimmer and the low end pound with a more clearer bass sound. I might be a long shot off, but to my ears, higher bitrates of 256 and 320 sound bass heavy music (hip hop, rap, electronic, house, techno, most modern rock, that sort of stuff) while keep high end clarity and 192 is sufficient for most other rock music. But if you're at it, might as well keep all the music at 320 and not be worried about quality, yet you would have to worry about drive space. Caveat emptor.