I've been using Kazaa for P2P tunes for a while now...over a year and a half.

I've been consistently disappointed with it.  The paid version is supposed to be ad-free, but I found that to not be entirely true. 

It not only slowed down my PC a noticeable amount, but I kept getting corrupt files that wouldn't play or played badly.  It really ticked me off....it wasn't the swiftest service in the world to begin with, but when I try to play a song that took me 10 mins to download and it won't play....yeah, that irritates me.

So, I was sort of looking for a new music source....and I came across Morpheus. 

I started with the free service....it's faster, more user friendly, easier to use (I guess that kind of falls under 'user friendly' huh?).....and this is all the free service, now, I haven't started to pay for it yet!  I've downloaded 5 tunes in under 4 mins, and they all play, and no pop ups as yet either.

Waaaaaay better than Kazaa so far.  Miles better.  Highly recommend it.