It's great working at a place where customers love the product. Not perfect of course, but it's great to continuously hear from customers about how much value they are getting in the product (the ROI is right in the analytics for everyone to see). Even better, high confidence in the CEO and two co-founders.
Cons
Same as any startup (even though the company has been around for 10 years with a really good customer base): things move fast, product evolves fast, processes will need improving as the company grows. So if you like startups, you'll love it.