Way, way late in replying to this thread--I blame it on being way-way late in showing up here altogether -- but I was immediately impressed to see it as a praised application here with a significant discount offered. Having first discovered TB when it was in version 1.2x or 1.3x something and having used it as my primary e-mail client for most of the years between then and now, it has a special place in my heart.
I was amused to read in the review about the complaints of seemingly trivial version increases -- having been there at the time of it. Version 1 supported free updates for several years, with most of the active users fearing version 2 was a pipe dream. When version 2 was released, even though only a minor upgrade from the current 1.x version with most of the new features promises rather than implemented, I had no problem paying--simply because I'd used it for so long without having to pay further. Version 3, on the other hand, came quickly and with a similar level of upgrade--trivial with promised features.
The developers are certainly not renowned for their internal organization and if they're known for their documentation, it's because it's absolutely terrible -- but there's no question about it, they deliver the most powerful e-mail client on the market. It's a lot easier to get your feet wet with Thunderbird, but even with dozens of extensions installed--neither it nor any other client can compare to the featureset of TB.
Anyway, it's nice stumbling into a public venue where she's getting some love.