I think it's a neat idea. I've used it with Dell and HP for support, and it's really a nice service.
That having been said... Who will man it? With Dell and HP, there are zillions of people on their payroll just waiting for notification that I want to chat. When I click the link on their site to chat, it grabs one of those people and we immediately start chatting about whatever the issue is. So, if you wanted to use it for your site, you'd need to be in a position to be ready to start chatting at a moment's notice at least during your business hours.
I can't justify the potential for distraction in my mind. You could be neck deep in an important task, and then some teenager pops in to ask if you can help proofread his homework for free. There goes your train of thought (if you are like me).
I would stick to normal IM programs (MSN, Yahoo, Skype) and only offer them if the customer makes an appointment. Then, I would consider that as billable time. Otherwise, with the on-site chat I think you may be opening up yourself to too much potential interruption.
Unless you mean you would only allow it to be used by appointment. Which...I don't know if that's possible out-of-the-box. But, if it is then nevermind most of my argument.