Sparrow has a deceptively familiar interface, after installing the Developer Preview of Lion. It is almost like they had some inside information regarding the changes in user interface. To me, however, their client doesn’t make sense. Its preview pane fills only to the right, it is not configurable. Sparrow lacks support for multiple mail accounts, and cannot interface with anything but Gmail and Google Apps accounts. For me, this is all well and good, as I use only Google Apps, but for customers who need to access more basic (or proprietary) email systems Sparrow isn’t an option.
Wait for Mail 5.0. It is worth it.