I don't know, I haven't got to use it yet

But as far as I am aware it is very similar to a video call in Skype.

Skype on the iPhone (and Android I think) doesn't do video calls yet.

Facetime only works on wifi at the moment, though I fully expect Apple have plans in place to resolve that in the future. I suspect it isn't technology that is holding them back on that, no doubt the carriers want some way to charge us again for the data it uses...

Edit:

And to be honest, I don't expect to make use of it really. I was just wanting to try it out of curiosity. I've made about half a dozen Skype video calls ever and one of those was last night so I could see the screen of my Mum's iPod to try and work out why Facetime wasn't working