Pros
The monitoring domain is full of unsolved problems. In particular when it comes to data analytics. IT operations is getting much more complex, and long standing system modelling and prediction problems are still unsolved in the domain.
Circonus brings some of the most powerful data analytics methods to the table, and is dedicated to advancing the status-quo even more. As a data-scientist it's a very interesting place to be (if you like hard problems).
There is an incredible amount of low level system engineering knowledge and competence in the company. Core components are written in C, work on multiple UNIXes, and are crazy performant even on mediocre hardware. I have profited a lot from open sharing of ideas, and general willingness to help. I learned a lot since I started here.
No bullying. No blaming. No ducking away. There is honest dedication to delivering value to the customer, from the management to the engineers. The CEO even contributes code himself. True "lead by example".
Cons
Expect to work independently.
Expect to deal with large grown code bases.
Expect to read a lot of code.
Small company.