Some kind soul sent me a GMail account as a thanks for my ham handed port of Synergy to OS X. I've been playing around with it for the past few days, and, having never used a webmail service before, I'm pretty impressed.
As a test, I signed up for the rather high bandwidth Linux Kernel Mailing List (around 200 messages per day, I would guess). The 'label' based sorting is pretty spiffy -- there are no folders, but you can create arbitrary labels that become groupings of messages. Filters applied to new mail can apply labels based on sender, etc, as you would expect, but one message can be mapped to a number of labels. The sorting possibilities are endless, and given that this is a Google production, search is Googlerific.
Threading works very well with messages being shown in a 'conversation' view -- all messages in the thread, including both sent and incoming mail, are shown in chronological order. However, portions of the conversation that have already been read are shown as thin lines preceding the full, unread messages, providing the illusion of a filing cabinet showing only the unread, relevant material. I think Edward Tufte would agree with this 'escaping flatland' realization of a pseudo-3D interface.
Of course, the coolest thing is that my account usage is growing by less than 1MB per day, so I could receive the same volume of e-mail for the next 3 years before needing to clean house. That is piece of mind for my lazy ass.