Hows the battery life when not in use?
Can you remain online with the app completely dormant, receiving calls via Push notification (the way iOS apps are supposed to work and the way most IM apps do)?
Or does the app keep chewing CPU and battery even while in the background, unless you log off so you cant get incoming calls? That seemed to be the case with recent versions of Skype: by many reports it wasnt using proper iOS multitasking, and battery life was impacted even when not using Skype. I tried it for a day or two and seemed to have the same problem. Rather than keep testing, I ditched Skype for many months while awaiting a new version.
This is a crucial point, along with how much battery does it use when actually in use. Skype on OSX uses a lot of energy to do what it does.