i like it a lot more than WhatsApp and IMO has a lot more potential. the 'delivered' and 'read' notifiers like BBM (for BlackBerry) has is awesome. only thing they need is groups and they may be able to takeover the IM scene.
I want to say someone in the forums said the reason you haven't seen any IM clients in Marketplace (AIM, Yahoo!, Trillian, etc.) is because WP7 doesn't have socket support. So if that's the deciding factor then we probably won't see this until Mango comes out.
Developers needed the API's ect that Mango are briging to make these apps. IM+ is out right now in the marketplace and they are using their servers to push notifications, but it won't work like you are used to until Mango.
Expect to see a bunch of IM apps this fall/winter.