Mailplane 3 just came out of beta, and it's great. I've been using the beta for a while, and saw stability and speed improve with each iteration.
While Gmail works great in a browser, there's something about having a dedicated application, as opposed to a Chrome SSB or Fluid instance. Mailplane actually is a wrapper for Gmail, but it's a bit like having a browser just for Gmail with access to many OS X goodies and 3rd party add-ons.
Before Google added multiple account switching, for example, Mailplane allowed you to easily switch between different Gmail accounts (it still does,
