Most support is now dome through our ticket system. Our support team replies usually within 30 mins during sane hours. We get raves about our fast CS replies so I think dropping chat/IM support has actually helped us.
Asynchronous support just scales better IMO.
the only unique thing that live chat will give you is the immediate 1:1, human-to-human, non-scripted ability to answer questions intelligently and close sales.
that is a huge timesink that you won't be able to keep up with forever, which will probably mean that you'll either need to scrap it, or outsource/script it. and either option moves away from the original intent, so why not start with a system that scales and just use whatever generic live chat option you want until you end up having to scrap it?
I really like IRC because it's not 1:1 - sure, I'm in there, and so is nick and will, when we have a chance, but there are many customers (right now, north of 40) in the room, and often they help oneanother.
It's also great for one off quick questions