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.