There's really no end to what you can do. You can pull as many servers into the mix as you like and make it as easy or hard as you see fit. And you can record the entire session so that you can go back and compare different candidates at your leisure. Or show them to your boss or a co-worker if you're having a hard time making a decision.
I do find that complicated situations often take a lot longer and you'll be on the call a needless amount of time. In the beginning, I'd stick with specific tasks that are fairly obvious to the level of person you're trying to hire.
If done right, this could be a new wave in interviewing techs. You can actually get past that barrier of question and answer and see for yourself what you're getting.
I've loved my experience with it and I hope you do too.
And really, give ReadyTalk a chance if you don't already have an online conference provider. They're cheap and easy to use, and all their support is U.S.-based.