I started using Postbox Express because it supports Omnifocus, but now I'm eyeing Postbox itself, which is a $39.95 application. Given that it's a variant of Thunderbird, what features does Postbox provide for that $40 that Thunderbird 3 does not?
For one thing, Postbox is a Cocoa application. Thunderbird isn't. So you can take advantage of some features you can only do in Cocoa applications. Such as pressing Control+Command+d to get an inline definition of a word. And in my opinion, Postbox looks one heck of a lot better than Thunderbird.