I've been using Sparrow for over a year now. It seems like they've stopped releasing updates, and lots of users (from what I'm reading on the App Store) are complaining of stability issues, but everything's fine here.
They were acquihired by Google. AirMail is a decent alternative though I find it a lot less stable than Sparrow, but use it because of the number of glaring problems Sparrow has that will never be fixed at this point. In the end I find AirMail a tad more usable.
