I have done it on 56k connection loads of times and it was a bit laggy, i.e. you would hear what you just said about a second later. You can't really have a decent conversation though, you can't talk over your partner, its a bit like radio calls, you have to wait until the other person has finished the sentence they are saying, gets weird if you talk over each other, lots of hesitation and repeating. If you have a quick connection though your laughing, bobs your uncle. (I have broadband now but not tried it yet, if anyone wants to 'test' it, get in touch and let me know and we can have a test conversation thingy.)
Quality is excellent, clear, no hiss/background noise etc all you have to do is configure your mic and sound level and that is a piece of cake to do, its simple really. I would consider a better option though like connecting to a mobile network that gives you free calls, mine gives me free calls to a landline anytime off peak etc so maybe something like that would be worth considering, also would you be in a public place such as the school/uni library? Not very personal then is it if other people are sitting next to you, it would be easier to go into chat and type away. Go for it though, have fun