Even with the upcoming overhaul to Mail in Lion, there are still plenty of alternatives for accessing your Gmail in OS X without going through the default Mail.app. Several TUAW bloggers are ardent fans of Mailplane, a US$24.99 app that turns Gmail's web interface into a dedicated email client.
Then there's MailTab, which comes in two flavors: Free and a $1.99 pro version. MailTab sits in the menu bar and allows you to access Gmail's mobile interface without needing to access a web browser. It works fairly well and brought in my Gmail information automatically from where I had been signed in with Safari. If you're familiar with Gmail's mobile interface, it's easy to use. The only difference between the free and the pro version is that Gmail chat is added to the pro version. It also lacks switching between linked accounts. You have to completely sign out of MailTab to sign into another account.
If you don't care to constantly fiddle with a web browser or have a single Gmail account, check out MailTab. But, if you have more than one Gmail account and want a more robust features that integrate with OS X, Mailplane is the better choice.