Uncomplex has released Mailplane 3.0, a major update to the Gmail-specific email client that now supports Gmail’s new compose mode and integrates Google Calendar. Mailplane 3.0 supports up to 10 accounts and enables you to switch instantly between tabbed accounts. It can also now preview attachments with Quick Look, as well as automatically compress directories dropped onto Mailplane as attachments. Other additions include support for Retina displays and for Notification Center (including the Do Not Disturb feature), plus inclusion of plug-ins and 30-percent discounts for the Gmail-enhancement services AwayFind (helps urgent messages find you) and RightInbox (for scheduling and tracking messages). ($24.95 new, 20-percent-off upgrade discount or free upgrades for purchases after 1 October 2012, 3.4 MB, release notes)
Anybody tried this version with Snow Leopard?
No, but I believe that it is specifically intended for Mountain Lion. Their home page gives the VERY strong impression that MP 2.5.4 I think is the one you want for 10.6.