It was a little surprising that iMessage wasn't rolled into iChat pretty much at it's advent. It made little sense that what is essentially an instant messenger integrated between iOS devices would not have a desktop client of any type. This wasn't entirely unexpected however; FaceTime also had no desktop presence early on, and then got a stand-alone app rather than iChat integration. As it stands, Messages for Mac follows a pattern of synchronizing terminology across OS X and iOS. Messages houses iMessage and SMS/MMS on your iPhone and now it houses iMessage and iChat on your Mac. iCal is to become Calendar, Address Book becomes Contacts. It's a sensible round of homogenisation. The same name, the same deal on all devices.
It makes for a fairly dull review to say that Messages kinda works exactly as you'd expect it to, but in honesty it does. It has a couple of differences and there are a couple of aspects to its operation where the bug or feature question is hard to answer. Previously with the FaceTime beta, the app went live in virtually identical condition to it's beta offering, so it's hard to say if Messages is likely to change significantly between now and July. The interface is clean, in fact it's so simple that I think it's probably the best IM interface across any service. It looks a whole lot like messages for the iPad.
One really nice advantage is that as you type in a contact name in order to initiate a discussion and their addresses are filtered out of your contacts, each address is polled for iMessage compatibility and updated in real time. You can see exactly which of your friends addresses are valid recipients. On iOS you have to pick one and wait for the send button to turn blue, which for address-gluttonous friends can be a hit and miss process if your memory isn't good (a me.com address though is often a safe bet). This is not a flagship feature but it's a great detail.
The most significant issue that I have with it is the way in which it grabs conversations from the ether. If you clear or delete a conversation, the next time you launch the app it will reappear in full. This seems to cause some trouble with unread message badges (almost certainly a bug) but is also just kinda annoying. Clearing the conversation caches does not seem to help. Presumably the conversation is being retrieved from the iMessage servers. The re-opening of a long conversation has temporary but noticeable impact on the performance of the app.
Receiving an iMessage will trigger Notification Center on all your relevant devices with the accompanying beeps, whistles and buzzes. This may bug some folks. I didn't mind too much. On the whole propagation of the read-status across my devices was prompt (though not always), which prevented repeat notifications. Something to be mindful of - especially as if you're like me your iPhone and iPad are rarely far from you and your Mac when you're about the house.