Download the latest versions of the best Mac apps at safe and trusted MacUpdate. Download, install. DejaLu, from the developer of Sparrow.
Setting up SparrowSparrow dispenses with both the Gmail and the Mac Mail interfaces and goes for a very streamlined look that users of are familiar with. Messages are presented in a narrow column with a button on the bottom right of the panel to expand a preview pane. You can start off with just one account, but it's easy to add more through preferences. You toggle back and forth between accounts by clicking on the small icons. Like iChat, you can either set your own icon or if you already had one associated with your Gmail account for chat purposes, Sparrow will import them. What Sparrow won't import, however, are your signatures. You get a default app signature instead.
That's all well and good, but I have specific ones that I use for various accounts. This meant going back to Gmail and copying over my signatures. It's not a deal-breaker, but a small hassle that I hope will be fixed in the future. Sparrow supports aliases, which you also have to manually add to the account you use the alias with. You also have the ability to set up custom shortcuts to show/hide Sparrow and create a new message. For those happily wed to Gmail's shortcuts, you can set those to override Sparrow's so you're more comfortable. Best of all, Sparrow gives you the option to turn off the ad that normally resides at the top of the message panel.
Using Sparrow Like Mailplane, Sparrow syncs with Gmail, so while you have messages on the Mac, it doesn't wipe them from the Gmail server. It makes it very nice for when you're using other programs like Mail on the iPad. I can read my mail on the iPad and it'll mark as being read through Sparrow without having to re-download the message -- my biggest pet peeve with Mail via OSX. If you have a large inbox and don't want to synchronize all of it, there is an option to download messages on demand, which limits the synchronization. With the preview pane minimized, you can double-click on a message to show the entire thing.