We use phplive on our site BUT I just tend to use it for monitoring these days rather than actual live help.
We found that when we had a button on our site saying click here for help/advice we ended up spending half our day with people who just wanted to chat about mens underwear and then not actually buy anything
So it was a cool feature but a bit of a time waster, if anything i'd implement it NOT to have a button but still use it so you can pop up and offer help if it looks like it's needed