Pros
The client base is not FTSE100, but real successful (generally) SME's and mid-corporates. This means the work is always varied and interesting (client engagement teams of say 5 to 30) rather than 100's. I left a smaller firm once qualified to work with larger international and private equity backed clients, but not big 4 because I like to have a bigger part in the work. All of this means BDO attracts the smart but independent thinkers (including those that don't fit in at big 4) so the atmosphere is fun, progressive and collegiate (at least my team was, when we all arranged to go for a few days on a Spanish Costa from our own pockets just to celebrate smashing budget!) This only worked because our partner was excellent. Smart guy and great developer of people (we all left over time to become FD's, not ideal for him but he's been great!)
Cons
Pay. Being different is hard: there isn't the bounty of fees that big4 get to pay top salaries, but variable pay is poor too (unlike smaller firms or boutiques who will pay large bonuses in good years)