I had Zopim for a time as well, and was very happy with it.
However, from the other side of the coin, are you certain you are going to have someone there for the majority of the day - every working day - for "live chats"? The number of times I have been to sites that offer live chat, but there is never an "operator" online to help, makes the whole thing pointless.
Never offer something you won't be able to keep up with.