The Apple iTune store is very easy to use and has a decent selection, although is missing many major artists.
The audio quality of the downloaded songs is okay, nothing great. They are in 128K AAC, which sounds a lot like 160K MP3.
I performed a comparison of the following (all using the same 2 songs):
1) Downloaded 128K AAC
2) Ripped 128K MP3
3) Ripped 192K MP3
4) Ripped loseless
5) Actual CD
I used a Sonica USB to digital converter to run the output from my Apple Powerbook into my $2000 Parasound DAC. My CD transport runs through the same DAC.
The worst was the 128K MP3. Had a hashy, boxed-in sound. Listenable as casual background music but it annoyed me if I paid attention.
The Apple 128 AAC was better, but still had a compressed, slightly hashy sound. Enough better that I could listen to it, while not expecting audiophile quality, and get past the compressed nature, but definitely short of CD-quality. I had my wife run the switches of my DAC to switch between CD and AAC and could pick which one was which almost immediately.
192 MP3 was just a touch better, in some areas, than the 128 AAC. Note this was not a converted AAC file to MP3. Haven't tried that.
Loseless through the powerbook was pretty good, but still not quite as open and natural as playing directly from my CD player. Good enough to do the trick unless one is in serious critical listening mode.
My overall opinion of the Apple downloads is that if you are going to use medium-priced headphones in a mobile, slightly noisy environment, then they are acceptable. In a lower-end home audio system they may also be quite usable. In a higher-end system, I would only buy songs if you really want a copy of a song and don't want it so bad that you want to drop $12-$15 on a CD.
Older songs that weren't recorded all the well to begin with, will show less degradation and thus be a more inviting target to buy this way.