As I am traveling since a year I ripped all my (legally purchased) cds into itunes BUT unfortunately I used the build in encoder. Now I am using the open source encoder Lame and had access to some of the same cds I originally ripped - what a difference. as soon as I get access again to my cds (all in storage) I will re-rip them.
So if you plan to get rid of your original cds make sure you are happy with the digital quality; mp3s are not even close to cds (or even good ole venyl) BUT there are quite some differences within mp3s ...
Cheers
Here's a good article discussing this issue:
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Quote from the end of the piece sums it up nicely:
"Basically, if you want true CD quality from the files on your iPod or music server, you must use WAV or AIF encoding or FLAC, ALC, or WMA Lossless. Both MP3 and AAC introduce fairly large changes in the measured spectra, even at the highest rate of 320kbps. There seems little point in spending large sums of money on superbly specified audio equipment if you are going to play sonically compromised, lossy-compressed music on it. It is true that there are better-performing MP3 codecs than the basic Fraunhöfer—many audiophiles recommend the LAME encoder—but the AAC codec used by iTunes has better resolution than MP3 at the same bit rate (if a little noisier at the top of the audioband). If you want the maximum number of files on your iPod, therefore, you take less of a quality hit if you use AAC encoding than if you use MP3. But "CD quality"? Yeah, right!"