I work for a small boutique and have worked for a big company too.
Pros:
- Much more interesting work
- Get to know people better / stable teams
- More control over your own diary
- Less travel
- More varied work
- Get to deal with real people (not 6 management layers removed from the client)
- Partners with big egos don't last unless they have a darn good reason to justify their ego
- Great work-life balance
- You learn useful skills as opposed to being a generalist / small cog in a big machine
Cons:
- Less pay
- Bad bonuses
- More personal environment makes it more difficult to ask for raises or move on to another job (hard to explain, but you feel like a traitor for doing that and get nowhere anyway)
- When politics get bad, there's no escape
- Being judged by fools who don't look beyond brand names and don't understand that people from small firms can actually be BETTER than those from larger ones (shock horror)