The problem with desktop email clients is that they’ve gone nowhere, and appear to be going nowhere fast. While Google has delivered solutions that make email less excruciating, Apple, Mozilla, Microsoft, and now Postbox are twiddling their thumbs and staring at the ceiling. The primary recommendation for a desktop email client is that you end up with a copy of your messages on your hard drive, and even that argument is moot.
I agree. Having recently switched from MobileMe+OS X apps to all online, Google-based services, I like doing mail online.