Frankly, wow. Gmail is very cool. In fact I possibly prefer it to mutt, my usual mailreader - but give me a couple of days before I pronounce on that.
Gmail is the first web application that actually deserves the name. For example, the inbox page has 2 lines of HTML, all the rest looks like:
D(["v","108424e99f735b5"] ); D(["i",0] ); D(["qu","0 MB","1000 MB","0%","#006633"] ); D(["ds",2,0,0,0,0,0] );