I just heard about this over the weekend and have already purchased and switched from the included mail client on my phone to Sparrow mail app.
It continues to amaze me what you can get for $2.99 on the App Store.
Rather than give you a long running text review of what sparrow does, I will let Thomas Houston from the Verge show you. Needless to say, I bought this on a recommendation of some other geeks who were thrilled with the better interface. I am using it because of the numerous other features you will see in the video here.