I think with a good MP3 encoder (i.e. LAME) and enough bits (~192kbps VBR) I find the sound is fairly transparent to uncompressed. Add in the extraneous noise in the car and it's even more of a non-issue. When I was primarily targeting my 2GB MP3 player I would often encode at 128kbps and even those sounded surprisingly good for what they were.
That said, I still buy about 99% of my music in CD form. Many MP3s I've bought simply don't sound good because they chose to use a bad encoder. CD is more future proof. I buy an MP3 and I'm stuck with MP3. I take my CDs and save them to my media server in FLAC (it's a lossless audio compression format). Now I can reencode those files every X years to meet the format that is currently in style. Plus I have a backup in the form of the CD.