I have yet to find anything that works as well in the areas I prefer Sparrow for—namely, ease of rapidly labeling and archiving messages. I use a number of labels for just about every message, to organize messages from/regarding each particular client, etc., or all my order receipts, or web service registration notifications. As such, Sparrow's contextual menu-based labeling made this really easy, and the ability to Archive and Label is something I haven't seen any other desktop client do, which is really unfortunate, because it saves a lot of time when going through a long list of email. Additionally, it's one of the few desktop clients which separates the concepts of archiving and deleting (going through Mail.app, it's very annoying to do actual deleting, and you have to delete it multiple times to get a message really gone).
Postbox is the closest I've found, but it doesn't get all the above correct in the way I prefer, and which Sparrow implemented well. I've bought a license for it regardless, in the hope that I can perhaps get used to it when Sparrow ends up dying down the road, but… I'd prefer a few refinements to the labeling and archiving process.