Vantaggi
The owner really puts care and attention into the people side of the business-- if there is a skill you want to learn or a personal accomodation to make your work life better, they are all ears and usually all action. When I was there, we had some great, juicy clients to work, so the work was always interesting. Lots of opportunity to learn people management skills.
Svantaggi
Simply due to being an agency, there is always a revolving door of folks at the lower levels needing nurturing, and it can be a bit of a gamble whether those entry-level folks are a fit for agency life or a career in PR at all. Also, there can be extreme ebbs and flows in workload (though lots of care in planning is done to try to flatten those out). The only cons are really down to the simple fact that it's an agency-- and PR pros tend to either like or not like that environment.