IMO is a IM app for both iPad and iPhone that sports a fast and simple interface, voice messaging and support for the majority of clients.
IMO is the highest rated app in the App Store for IM and is completely free with no features hidden behind in-app purchases.
Pro Beautiful user interface
Although IMO has a comparable, and in some cases superior, feature set when compared to the competition its key selling point is its interface.
IMO also has a fully functional website that you can use for chat while on your desktop or laptop. This really puts IMO ahead of the competition as you can use the same system on all your devices so you don't have to deal with overlapping notifications etc. (Note: IM+ also has this feature)
It should also be noted that it has the highest rating in the App Store due to both the UI and its stability.