If you have a high-capacity iPod, I see no reason not to rip at anything less than Apple Lossless, unless you have a large CD collection and are bound and determined to have most of your collection on your iPod. Apple Lossless is exactly what its name implies: it's compressed but lossless encoding, meaning no data is lost when you rip the CD, even though the file sizes are roughly half the size of an AIFF or WAV file, which are uncompressed. On my measly 30GB iPod I have one each of all the Mahler symphonies (of course!), as well as all the symphonies by Shostakovich, Sibelius, and Tchaikovsky, plus all of the 'Ring' cycle and at least 5 other complete operas. All this with room to spare for lots of other pieces. And all of it is encoded with Apple Lossless. It sounds great. I also have an 8GB iPod Nano, and there I've chosen to use 320kBps AAC (which many say sounds better than MP3 at equivalent bitrates, though I haven't done any comparisons), and it sounds quite good as well, though perhaps not quite up to the level of Apple Lossless.
Russell