If you're using OS X Lion, you can now download Messages Beta, which was made available yesterday afternoon.
You have to have the latest version of Lion - 10.7.3 and yes, it includes the last massive 1.34GB update. So if you haven't done that, or if you're on Snow Leopard, you won't be able to try it.
Messages will replace iChat, and have full support for iMessage (so it will be able to send messages between iPhone, iPad and iPod touch running iOS 5).
I used to use iChat as my IM client, and it takes some time adjusting to the new look. I prefer the list of contacts on the left hand pane to be a bit narrow. Your contacts will appear in reverse chronological order, unlike in iChat were you could choose the order.
All chats from your Mac and iDevice are synced, unless you don't have an email address attached to iMessage. I didn't, and then last night I added it, but I felt it was too much to have chats from my Mac syncing immediately on my phone. So I removed it. Also, I don't use iMessage on my iPad.
Anyone tried Messages Beta yet? What do you think? You can download it from here.