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 you 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 in height. 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 added it, but felt it was too much to have chats from my Mac syncing immediately on my phone. And ultimately, waste of battery on my phone. So I removed it. I also don’t use iMessage on my iPad.
You can download it here.