Pros
I have interacted with a lot of different staffing firms from both sides of the fence and in my experience, these companies run the gamut - from borderline scammy to spectacular to work with. 3P falls into the "spectacular to work with" category. First, they want to know the people they are representing so that they can speak knowledgeably about a candidate to a company and make good matches. They didn't just throw any job my way - they knew what I was looking for and cared about making that happen. Second, they were always honest about what they had in the pipeline. They initially didn't have any openings for me, but they were so good about regularly staying in touch to check in on my job search. Because they knew what I wanted, it didn't take long at all for opportunities to materialize because they had a really good idea of what I was looking for and where I would be a good fit. Third, they made themselves available at all hours as I progressed through the candidacy process so that if I had any questions as I was making my decisions, I could get answers right away. And finally, they were great about preparing me for each step of the process - there were no surprises.
Though I was typically in contact with one person, they were scheduled to go out of town for a week so I also had the chance to work with some of the other staff. To a person, they were equally knowledgable, attentive, friendly, and concerned with connecting me to an opportunity that would be a great fit. I couldn't be happier with my interactions with 3P.
3P is on the up and up and their business practices will make this company very successful. I plan to work with them the next time I am in a position to hire as well, because I trust in the way they conduct themselves.
Cons
I don't work in the 3P office, so I can't speak to the culture beyond what I have seen when visiting. I do not have any cons to note.