Pros
You'll work alongside bright, talented and ambitious peers who often see the big picture and have good instincts on how to achieve success when faced with significant obstacles.
Cons
The bane of Yahoo! is senior management. They're a tribe of executive nomads who roam between companies looking to loot as much as possible until they either leave of their own accord or are forced out. I attended a company all-hands meeting once where a brave soul stood up and verbalized what I held inside for the better part of 5 years... at Yahoo!, the only criteria for strategic direction is that it be significantly differentiated from that of the former regime. Plenty of accountability at the lower levels, but executive management gets endless free passes.