Upon launching the app, you're prompted to enter your Apple ID (or create one if necessary) you can then choose whether you want to turn on read receipts and add additional email addresses you can be reached at. The install process was a little less elegant that we've come to expect from Apple and required a reboot, but was pretty straightforward in the end. In a quick test, Messages on Lion seems to work much like it does when running on Mountain Lion. This previously was an iOS-only affair, but now you can send messages seamlessly between your Mac and iOS devices using your iCloud account. Of course, the big new feature (aside from the redesigned interface) is the integration of Apple's iMessage service. If you can get it to work, it's a quick 66MB download. Some of us were able to download the app, though others have reported errors when visiting the Messages page on Apple's site. If you're interested in giving it a shot, you'll need to have upgraded to 10.7.3, which rolled out with some difficulty a few weeks back.
A beta version of Apple's new Messages app, which will be part of OS X Mountain Lion, is available to download now from Apple's website.