Yep, pretty much this.
In all honesty it will all depend on how you actually use it.
Put it on a website that is bad anyway, then you will probably not get much out of a system.
Place it on a site that already works fairly well and use chat to "compliment" the site, then yeah, It will likely work better.
We use imsupporting for ours and it works quite well.
You just need to make sure you are actually online, able to help your users fairly often. Whats the point otherwise?
For example. We were fairly sure people were leaving our site and not booking viewing due to problems with our site itself ( not having enough info etc. ) After we installed chat, we slowly understood what our users were having problems with. It helped us fix our issues and in all honesty helped our conversions so to speak.