Pros
A small company means you will be doing a lot of things. My employee gave me lots of opportunities including presenting in conferences about the software, contributing to the software's development, helping with project execution, after maintenance. I got to travel to client sites to do a post project implementation review, which was a lot of learning.
Cons
You have to learn things fast and without much training.
Have to be self-motivated to find out things that sometimes there is no documentation and no one knows about.