I've used Zopim for around a year and have never been disappointed in any of their services/features.
Personally, the price per agent is worthwhile if you're going to be optimizing all the time possible on those agent accounts. Furthermore, if you are going to be dealing with a small number of chats, then I wouldn't even suggest using more than 2-3 agents. One agent can easily handle 2 calls at once if needed, if not more.
If your support is good, then Zopim will pay for itself.
Personally, the price per agent is worthwhile if you're going to be optimizing all the time possible on those agent accounts. Furthermore, if you are going to be dealing with a small number of chats, then I wouldn't even suggest using more than 2-3 agents. One agent can easily handle 2 calls at once if needed, if not more.
If your support is good, then Zopim will pay for itself.