Recently, the DownloadSquad blog made a posting about a mail app called Postbox. Its idea is to tie into social media, with the ability to pull in Tweets, Facebook status updates, and FriendFeed postings, as well as support for POP3, IMAP, and MobileMe email.
As an email client, its perfectly acceptable. To be blunt, it's Mozilla Thunderbird with a new skin. It is based on Thunderbird, so any quirks or favorite elements you had will still be there. The notable exception, though, is that extensions are unavailable. A nice addition is the ability to view GMail messages in a threaded style similar to the web interface, but this may have been recently added to Thunderbird, which I haven't used in several years. Also nice is the ability to Google search from the interface, and a listing of all the links and embedded media in the current message.
The social media aspect, however, is terrible. You can make updates, but not receive them. Making updates is done through an arcane (and oddly-named) "Promote" dropdown button. For other features, like To-Do tasks, there is no interface for easy viewing. To-Dos can be created from emails, and then pinned to the top of the email list, but there is no central store for them.
The threaded GMail messages is an awesome feature, and one I've been waiting for in a desktop client since GMail first added them. But the half-finished social networking features are so bad to be nearly worthless.
One suggestion: A split sidebar, like on the left side, added to the right. One section should be social media updates (Facebook/Twitter/FriendFeed messages), and another to-dos. Better yet, these panes could be added to the left sidebar. Such services work best at-a-glance.