Lisa Hoover has written a very nice review about Mailplane. She starts with:
I’ve been a hardcore Gmail user since the early days, when beta really meant the email service was in its infancy. It’s perfect for managing my email but there are two big things Gmail can’t do that seriously hinder my daily workflow: There’s no easy way to switch between Gmail accounts, and no way to drag and drop a attachments into emails.
And then she presents Mailplane:
To get around these two issues, I use Mailplane. It’s a fantastic application that replicates the Gmail interface right on your desktop and integrates so well with your Mac that you’ll never want to access Gmail from the web again.