Compete.com published an aggregate view of all US internet users engagement with web-based email clients. At first glance, there's some surprising data in there.
"Based on unique visitors from the US IBP (internet browsing population), Gmail doesn't even hold a candle to Hotmail (94% larger than Gmail) and Yahoo Mail (190% larger). Gmail's buzz is obviously much larger than it's bite," said Drew Fortin.
According to Wikipedia Hotmail was established in 1994, Yahoo! Mail in 1997, and Gmail in 2004, so there's certainly an opportunity for Gmail to gain market share as it matures.
Fortin also points out that web based email traffic seems to be slowing. In fact, traffic has decreased 11% year over year shown on this chart below: