I just got done implementing fail code for a client from LiveHelper. Stay away from them. Essentially the problem was the javascript they were using was extremely inefficient. Instead of keeping an object with a hash of customer id and whether the tech was online and polling every so often, each implementation on a page made the same requests multiple times. And you couldn't delay execution until after the DOM loaded because their javascript wrote code directly where the script was called, instead of maybe after a specific div, or replacing the contents of a specific div?
So our client has moved on to ProvideSupport and they seem better. I personally have no greater experience than that.
As for PHP based clients, not really sure if any are good. I'd rather put my weight upon a third-party that I can go to with complaints if/when something breaks than go "well, it's OSS so you get what you pay for" and have to waste my time fixing it.