Fast, stable, with lots of useful features. In particular, it leverages Apple Address Book groups; provides topics, templates, and response features. Gmail works well. However Postbox uses massive amounts of memory for an email app - typically over 300 MB on my iMac on OS X 10.6.8. Also, as mentioned in other posts, developer support is not a strong suit; made more of an issue because it's often difficult to find answers in the online help for the more arcane features. Once you get up to speed on how things work, though, everyday operations are a breeze.

I have a sole proprietor business; if I was part of a larger operation (scheduling meetings, etc.), I'd probably be using MS Outlook, which I have but don't use because it's slow and Postbox provides more features that I use daily. Apple Mail would probably do everything that I'd want if I were willing to explore 3rd party add-ons.

Large RAM footprint, but is much faster than Outlook when sorting email and searching. Works well with Gmail. Templates and boilerplate Responses are timesaving features. Online help is comprehensive but organization often makes finding information time-consuming. Lots of useful features that make competing email apps appear outdated by comparison, although conversations in Outlook seem to be organized more accurately. I'm not getting rid of Outlook anytime soon, but Postbox seems to be coming out on top as my default email app.

I contacted Postbox support through the company website a couple of days ago regarding an graphics anomaly, and received an email reply today that resolved the issue. I'd say that the developers are serious about good customer support.

My #1 must-have Mac utility. Two days ago, I isolated a potential conflict between DF and an application and emailed the developer; yesterday I received an updated version of DF that fixed the issue. Doesn't get any better than that.

Can't use it for now; v.6 currently doesn't work with Firebug, Devonthink or 1Psssword extensions.