Since hearing that Thunderbird is going to start collecting dust, I've been looking for a decent Windows client and was wishing Sparrow worked on Windows. So this makes me even more sad. I wish Google would do the right thing and open-source it like they did with Etherpad.
Postbox was my next solution. Is it better than Thunderbird?
I've been somewhat happy with Thunderbird actually, in terms of performance and stability. It's just that the whole codebase seems unwieldy (try developing extensions for it) and may not be around a lot longer.
I bought Postbox and really like it. They've based it on Thunderbird 2 which is the version I like--3 made some UI changes I don't prefer. By continuing to update 2's code, I get an up-to-date e-mail client that acts how I want. Plus, with a couple of settings tweaks, it can look just like Sparrow, it's available for Mac AND Windows, it supports Gmail's tags (I don't use Gmail so I can't check that), and even does the social network integration. It's also the same price as Sparrow (actually, 4 cents cheaper) at USD$9.95.
