Bakari Chavanu published a very nice review about Mailplane and another application called Webnote. Both apps solved some problems he was experiencing. With Mailplane he solved these:
Before I downloaded and started using Mailplane, checking my Gmail accounts was nothing short of a pain. I tried having selected emails in my accounts forwarded to my desktop Apple Mail client, but I ended up not liking that approach simply because I wanted to keep my Google accounts separate from my MobileMe accounts.Checking Gmail online was also not a joy because I couldn’t use shortcut keystrokes and the awesome time-saving SteerMouse plug-in to quickly navigate email messages in the online client.