Me too! Lightning fast. I actually run it still in XP Mode in Windows 7 (couldn't get it working natively) for my personal email (Thunderbird for work, like to have a clean separation), even with that layer of emulation it's the fastest thing ever. I also love the ability to alt-click on any field and have the messages grouped by that. Closest I've found in Thunderbird (penelope extension is awful) is the Nostalgy extension, set up so that ` toggles between grouping messages by sender or subject based on the email you have selected. Not as awesome and not as fast, but still super handy.

I would use it but I worry about using programs that are not being actively maintained because of the risk of security vulnerabilities. Especially in email where you can have a lot of risky inputs (spam, phishing attempts etc) - even though I have great spam protection (thanks pobox) there is still a risk. I'm not sure what I'm going to do on Windows yet... I'm trying Postbox and it's nice.