I upgraded today on my Windows 7 work machine. The overall email experience is unchanged--I use it to access my work account on Rackspace email via IMAP, and the new version continues to work fine. But overall I'm underwhelmed to the point that I wish I hadn't upgraded.
At first the new Windows 7 theme struck me as a nice improvement with all its pretty glassy transparency up top, but a few hours of hard use it's just annoying... the menu bar is partially transparent now, so if there's clutter or a dark background under the Tbird window then the top-level menu items lose some contrast and become harder to read (for my crappy eyes, at least). The whole UI lost some of its snappiness, too--I'm not sure if it's the new theme's fault, but resizing the Thunderbird window isn't nearly as smooth as it used to be and the menus feel sluggish when I click on them.
But here's the part about the theme that's driving me batshit insane: Moving the Thunderbird window is broken (it's an advanced feature, I know, but hear me out). There's this big transparent glass area up top with all kinds of empty space that's just begging for you to click on it so you can drag the window around. But the click only registers if you hit it on the very top part of the window. Every other Windows program with a big transparent glass area (including Firefox) lets you click anywhere on the glass to move the window. But not Thunderbird. Most of that glass is a useless no-man's land. (This is so contrary to my deep Win7 usage reflexes that I thought that my mouse was broken for a while.)
The highly-touted improvements to the tab management doesn't do much for me either. Sure, it's neat that I can detach tabs and move them around now, but if I compose a new message then it has to be in a separate window. I can't dock the composition as a tab anywhere, so I have to mess around in the Windows taskbar or Alt-Tab to switch between the messages I'm composing. I'm not sure how this oversight slipped through the cracks.
Oh, and half of my extensions are incompatible after the upgrade. This should annoy much more than it does, but I've grown numb to it over the years.