Which services do you use? I use AIM, GChat and Facebook and Beejive is by far the best client out there for those. Push notifications work flawlessly and the whole app looks like it was made my Apple and came built into the phone. You can send pictures, audio, etc. through the chat. It's wonderful, but it is the most expensive IM client out there. I've tried all the free ones and none of the others are even close, especially for the chat services I need.
PS: You might want to try the App Store section of this forum. Do a search there, there are many threads just like this one that go into further detail.