I had Olark on my site for a brief period to try it out. I really like the functionality that it provides and I think they are doing a great job.

That said, I ended up removing it after finding that the javascript service was rather unreliable and would cause delays for the loading of my site. I'm sure it was just a brief outage or whatever, but it does me no good if I can't get my pages to load.

I'll probably revisit it in the future as olark definitely had the best product out of all of them. The integration with jabber is key.