I've basically completed my migration to gmail from hotmail, and I'm already thrilled about the change. I'm a member of several mailing lists, and gmail keeps them all nicely organized for me. It also has a whole set of features, etc. Perhaps the thing I like most about it is that I can integrate my gmail account, my blogger account, my RSS feed readers, and other tools onto my igoogle homepage. I know MSN has some similar functionality, but I have to say that I really just prefer gmail.
Of course, time will tell which mail client is the best, but at the moment my money is on gmail. Let's hope it pans out!