As others have said, Zopim is free for one user. We have used it for over a year and it is really good except if I don't interact with it for more than a couple of hours, it seems to disconnect so I have to force reconnect - we have an old website so it might be only an issue for us.
I don't find it intrusive on my mobile - it acts differently and stays at bottom unless customer clicks and then it opens a new window.
We have found it very worthwhile. We think sometimes a customer would leave the site - eg because they are not sure which dog wetsuit size they need for a breed, but with Zopim, we can help.
The other good thing is when we are offline, it offers them the chance to leave a message - without looking for email/contact info. Quite a lot of people use this and we can call them back next day.
Hope this helps.
