GoogleMail is excellent for filtering out SPAM. Thats why i use it. Plus it's fast and no need to reload to see if you have got emails, It reloads using AJAX.
I don't find their filtering system perfect as often I see few of my important regular mail lying in spam list and some of the spam mail in my inbox.
Thats because the IP address of that server has been marked down as spam, everything sent from that specific server will end up in spam. It's all down to one idiot who sends about 600 unsoliciated spam emails, google adds the IP and voila.