FLAC is for the most audiophile of audiophiles
No really, for some content, like very subtle piano music (try the Goldberg Variations by J.S.Bach) you can definitely hear the limitations of mp3. With good headphones you can hear a metallic artefact at the beginning of each low note, even with 320kbps mp3 (at least that was when I last checked with lame 0.93 - maybe it got better in newer releases). However with aac I wasn't able to hear this, that's why I switched to converting all my cds to high-bitrate aac (there's two good, free codecs for aac: one is in itunes, the other is a free command line encoder by Ahead, the company that makes Nero)
With FLAC you can be 100% sure though you won't lose anything at all, because it is lossless. Not relatively lossless, but mathematically proven lossless (don't ask me how it works, but I believe the mathematicians who developed that algorithm). The downside is it only compresses roughly 2:1.
Another great audio player that plays just everything (and is capable of every conversion, tagging and whatnot you can imagine) is foobar2000. It looks very minimalistic, but it's a multi-talented beast!