Seth Smith just reviewed Mailplane on the AppYourMac site:
Mailplane offers lots of features to make your life easier when it comes to emailing. One of my favorite features is, the function of being easily create attachments by a simple drag and drop method. If you like attaching photo’s or other files, this feature really does come in handy. Another great feature of Mailplane is that you are able to switch between Gmail accounts fast and with no hassle. I myself have about four Gmail accounts and I now have them all synced with Mailplane, it is so much easier to switch between them now that I am using this application. Of course Mailplane stores all of your passwords on a keychain, so once again it saves your time.