[Let me say I love the concept of Olark. I'm just not sure how it plays out in practice.]
We get about 300 uniques a day and he hasn't been overwhelmed at all.
The biggest issue (IMO) is making the distinction between sales and support. We don't use Olark for support (and don't want to). But for sales, it's awesome.
If the cost of offering chat is < value you get from chat it works out.
The trick is to understand that equation for your business.
We launched targeted chat to help you focus your chats on just particular visitors. One could argue that the value talking to customers who land on your pricing page, while ignoring everyone else always works out.
One year of the Bronze plan, at $180, is several orders of magnitude lower than my average customer lifetime value, so it's easily worth it, even if it only gains me 1 customer per year. Your mileage may vary.
But right now, most of my customers are coming through my email marketing, so I won't signup now. But I definitely will in a few months.