Pros
Well-run company where you are trusted to get on with it and can create a role that you find interesting / engaging. I've worked at a range of agencies and this is by far the best run one. Very few pointless meetings - so you have time to think - and are less likely to be playing catch up after 5pm. Hours are very flexible - if I want to work out before lunch I can. Don't need to pretend to be working hard when you are quiet, but on flip side, when you're busy you'll work overtime. Because project teams are small, you generally have a lot of control of when those busy times are. Relatively flat structure so there's a lot of interesting discussion and everyone is treated with respect. Lots of people with 20+ years experience who you can learn from, and increasing numbers of competent researchers in their mid-20s to mid-30s who bring in new ideas. Very positive and kind atmosphere. Lots of light conversation, interesting opinions, writing puns on the fridge door etc. It has recently been bought out by a PE firm, but they've left us to it.
Cons
It's an older crowd by agency standards so there is not as much of a Thursday drinks culture - although some of the newer younger people are getting that going. Everyone is doing their own thing in their own way, or working in small teams on mid-sized projects. If you like being part of a big team with a clear role on big projects, this probably isn't the right place for that.