This is such a great question @aafable! Ideally we'd always say 'yes' to these invitations because you really never know whether or how you might work together until you have a conversation. On the other hand, we're all pretty busy. This coffee date could be worth your time or a waste of it, and you won't find out until you're sitting there java in hand. 

Totally agree with @Rustler on the "I'll get back to you," and @john-pero makes a great case for sussing out what to expect. You might try following up via email with "It was a pleasure to meet you at the Chamber. Can you tell me a bit more about your product? I'd love to learn more about your business as well." That way if you do decide to meet up it's clear that the coffee is more of networking opportunity, not a sales pitch.