Larry--
I've had Napster for 6 months or so. The selection is very very good, although you'll find holes in their catalogue right away. Generally, newer artists, newer albums are all offered in their entirety. Older rock albums are spotty or not offered at all.
Engadget.com has rated the Napster site superior to itunes in terms of navigation and selection. Napster has a browse function that works very well.
The tracks played perfectly in MC11, better at first than they did in Napster itself. The issue is the current MSFT DRM, which is a total pain. I run win2k and I found it to be VERY difficult to deal with. Latest XP is supposed to be much easier.
Napster catalogue is 128 kbps WMA. Yahoo is apparently 192 kbps WMA. There is probably a slight edge in sound quality to the itunes AAC, although I have not done an a/b.
If you sign up for Napster, bring plenty of patience. The customer service folks are enthusiastic and somewhat knowledgeable, but the whole field of DRM audio is being invented on the fly, so you will have to bear with. I got the idea that the troubles I was having with the service were fairly common, particularly with win2k and various anti-spyware.
Lastly, and this was the clincher for me, you'll find that artists will take some of their songs back without telling you. All of a sudden songs won't play and you won't know why. Neil Young has been partiicularly selfish in this respect, so he better not come around trying to borrow money from me, that's for sure.
Haven't tried Yahoo. I'm most impressed by the state of the art of streaming, which has greatly improved the last few years. All the DRM hassles, all the catalogue hassles go away, although currently you don't get to pick your tracks. I wonder what JimH was hinting about?