I wouldn't trade the time I had at OnSIP for anything. Not only will you be well compensated, you'll get the chance to work with some of the best software engineers. That's not something you can find anywhere. I left OnSIP a much better engineer than when I started and I can't overstate how valuable that experience is.
Cons
It's a small company and sometimes the politics can be strange, but I'd say they more than make up for it by their commitment to taking care of their employees.