Reguarding the question of making mix disks... is the goal a regular CD or mp3/wma type disk? If you are going to make a mix CD don't convert the files, just rip them off the org CD to WAV format and once you have all the ones you want you can then burn em back to a single CD. There are a million programs out there that can do it, very often your CD writing software can handle all of that, NERO does for sure. If you want to make sure you are getting very accurate rips use EAC (its slow but works great).

Compressed is a whole other can of worms. Of course if you compress something using a lossy codec (mp3/ogg/wma etc) you are going to lose quality, the debate comes in when some people claim they can hear the difference, some people can't and others just don't care.

Personally I'm from the side that believes if you use a quality encoding process it becomes near impossible to tell under most circumstances, plus the convienance factor is the deal maker for me. So in that case it becomes which codec. That again comes down to personal preference. It's true that mp3's at low bitrates are not as good as newer codecs (wav/ogg etc), but the differences at higher bit rates (256k +) are very minor. There is no other compressed codec out there that is as accepted as mp3 either (players, etc). I prefer encoding into mp3 using VBR (variable bit rate) using the LAME encoder's alt-preset-standard settings.

I still believe the bigger issue is usually how poorly albums are mastered. I think that has much more to do with sound quality then anything you might lose compressing music.