I think Skype's so well integrated into the N900 that I've received calls without realising that they're through Skype.

Batterlife wise I find that MSN is the biggest battery consumer. Sometimes MSN feels so congested that watching YouTube videos might prove impossible even though there's nothing going on with the MSN. But as soon as I log out of MSN, the phone speeds up again. I've been using Pecan but its been the same with all of the MSNs. Skype seems to be a very "light" IM account to have active at all times.

Video calling in Skype with the N900 doesn't look very good but functions well. In addition it works over 3G, unlike withe the iPhone4.