Pros
Good name on your CV. Lots of variety. Most of the work is interesting.
Cons
1) Big logjam of senior managers trying to get to partnership. Existing partners trying to restrict number of new partners in order to maintain high profit per partner (especially in UK, less so in other countries). Many existing partners are not as good as the senior managers that work for them so there is little chance they will leave and create spaces. So partnership dependent upon growth of firm and all the senior managers are fighting to win work in a declining business environment and it is getting quite nasty at times. This is made worse by partners from other countries transferring to UK because the compensation is better - they do their 2 or 3 year stint in Japan or Russia and then see UK compensation as their reward. 2) Compensation not aligned with ability or performance - if you negotiate your joining salary badly then you are stuck. Maybe you get a good review and your percentage increase exceeds your peers, but it still leaves you on a lower base salary than less able people who managed to join on a higher salary.
