I have Yahoo Mail (along with Yahoo Messenger, Yahoo Calendar, etc), Windows Live Mail (and WLMessenger), and Gmail, and of the three, I much prefer the Yahoo group. Yahoo seems to offer more options and preferences than the others, and a large memory, so that I can send and receive very large emails. Functions are easy, and I love the fact that I can set Yahoo Messenger to not only give me pop-ups when I get emails (and IMs too) but I also get a little icon in my system tray alerting me when I have unopened mail. The calendar works very easily with Yahoo Mail --- it's all very seamless.
Also, another very important issue is that my Windows Live email has been hacked quite a few times, but my Yahoo Mail has never been hacked! (Knock on wood!) I don't use my Gmail often, so that really doesn't count. So I'm thinking that Yahoo Mail might be more secure than Windows Live Mail.