Postbox is a feature-rich version of Mozilla Thunderbird. I had done a complete review of features of Postbox previously here. Postbix has finally come out of beta after 16 beta releases and is stable now. However, Postbox 1.0 is not going to be a Freeware and comes for US$39.95 for a license. If you want to try before you buy, Postbox has a 30-day free trial available.
If your question is “Should I get Postbox?”, then read on… If you just need a basic POP3/IMAP mail client to send plain and HTML mails, then Thunderbird is just fine. But if you are a power-user and want the Advanced search, Tabs, Auto-complete, ToDo lists and the cool Add-ons, then you can consider getting a license.
Personally, I think the price is a bit more and something like $19.99 could have been more appropriate. For that price I would consider getting Postbix and moving from Apple Mail.