Lossy compression audio formats such as MP3 can and will often provide a "lesser" version of the original data. How much impact there is depends on a number of factors - how much compression is applied or how good the components in the rest of the signal path are. Lossless compression should yield results that are very close to indistinguishable from the original CD.

Having said all that, the question really is what works for you. If you have good speakers and good electronics but also like the convenience of MP3's, then maybe you want to experiment with Apple Lossless or FLAC for lossless archiving. You may even want to go with lossy compression and use iTunes for casual listening but keep a CD player around for those occasions when you are doing more serious listening. Owl's_Warder summed it up well - do what you like. That's probably one of the best bits of advise in this hobby. Your ear and your tastes should dictate what you buy, not other people's opinions.

I've been using MP3 for music listening at work for ages now, and when I got an iPod for Christmas I tried hooking it to my 990 - there was a clear advantage to the original CD over the MP3 (better components are more revealing of the compression), but I've still got a cable tucked back in the equipment rack so I can hook up the iPod on occasion because it is convenient. Once I run some network cable to the equipment rack (probably next weekend, since we'll be in Chicago this weekend), I may even think about getting a device like a Squeezebox or Soundbridge to make use of that convenience. I'll still keep my CD's handy so I can toss them in when I have the opportunity to really sit down and listen to the system, but for those times when I'm doing other stuff and just want some background tunes I could see using the iPod or a media server instead.