TJ Luoma just published an excellent post on TUAW. It is a comprehensive account of the Mailplane 2.2 features.
Here is his favorite feature:
My favorite new feature is the ability to drag a Mailplane URL directly from the title bar. I use this with BusyCal's URL field if I need to connect an email with a meeting. It works for conversations or searches.
And the conclusion:
Version 2.2 is a pretty significant update despite its minor numerical increment.
Naturally, you can always use Gmail in a browser for free rather than spending money on an app like Mailplane. But I've used Mailplane since it was in beta, and I would never go back. It is very actively developed, with new features regularly being added. Having a real Mac app for a mail client is well worth the money for anyone who spends a lot of time in email. If you manage multiple Gmail or Google Apps email accounts, the easy switching alone makes it worth the asking price.