Good experience and great networking opportunities, but remember you are a disposable cog in the machine
Pros
Exposure to a wide variety of challenges, great smart coworkers, opportunity to learn. Fast paced and interesting. Never a dull moment. There are some truly caring and exceptional people to hang around with. Generally close to the cutting edge and it's impossible to become stale.
Cons
Tenure valued over talent for PPD admission. Butt kissing practically essential. Annoying minority of posers and hacks. Too many stupid "rules". Can often border on slavery and requires tremendous personal sacrifice (no weekends or holidays off if it's crunch time). Very much a "what have you done for me today" culture. Very unfair performance appraisal process: mediocre performers can get better ratings because someone feels guilty or they're being considered for promotion. It's frustrating to do a great job yet get a 2 you didn't deserve.