Vantaggi
You get to work alongside many traditional agency vets, who can tell you war stories from McCann, BBDO, all of the big shops. Small company (~25 people total), chances are you will be wearing many hats (taking on many responsibilities) and will have to try really hard or seriously antagonize some of the senior management to be let go. Also, the family-like work culture is very welcoming and they are understanding of periodical appointments/reasons to be late. Time off is relatively easy to get vs. other companies.
Svantaggi
90% of your workload is traditional media -- print, out of home, television, etc. You'll definitely be on your own when it comes to accumulating digital/online experience, save for relatively infrequent online campaigns involving relatively simple RMU flash animation. Being a small agency, and a multicultural one at that, budgets/expense reports/etc will be that much more closely scrutinized. Also, the overwhelmingly family-friendly/lowest-common-denominator culture of the main clients definitely trickles down to the agency -- if you're looking for "work hard, play hard", this is not it.