Pros
Brilliant technical staff as co-workers, with varied backgrounds in Philosophy, Linguistics, Computer Science. Like med school in that it is hard to get in there but, if they hire you, they will work hard to help you succeed. Management takes policies seriously and is fair and open minded, and has a very frank how-the-company-is-doing monthly meeting with all the staff. Very stable and conservative fiscally, not like your typical startup. Almost no hierarchy, everyone is just a member of the technical staff, period.
Cons
Sometimes I got pigeonholed and had to ask to move to a new project. They expect you to either know your stuff going in, in particular first order logic, or learn it. This goes to some extent for programmers, too, not just people doing ontology work.