I have used Olark in my past two companies' customer service departments (which I managed) to interact with customers in real time.
Pros: Olark is very reliable, has many well thought out features and has very helpful customer service (via chat of course) when you need them. Its very customizable, intuitive and quick and easy to set up and maintain. Olark's features include - starting conversations when customers preform specific behaviors, storing transcripts, noticing when a customer is back for a 2nd visit, and acts as a "contact us" box when your chat agents are offline. You can have multiple employees on chat at the same time - though you do have to pay by user. They also integrate into a number of other platforms such as Salesforce, Desk.com and Zendesk. Though these integrations are rather limited.
Cons: I would love to see them get a bit stronger on analytics - they don't offer much in that department but you can include them in Google Analytics which gets you most of what you need to know. It'd be great if their integrations were a bit stronger (or if you could use more than one at a time). It can also be tricky to use if you have more than 15 employees that will be on chat.
All in all I highly recommend Olark and have enjoyed working with them over the past years!