Pros
Best in class - biggest network of disposal facilities and most Fortune 500 customers are clients. You will learn as much as you want to here, and their safety record is second to none. (Safety First) is a very real adage here and held in the highest regard. Hourly personnel can move up the ladder to increased responsibilities (in the hourly class). Management, not so much. If you are new to the Environmental field, Clean Harbors is for you. If you get lucky, you might get somewhere someday. If not, you leave with the best training and experience that even money cannot buy!
Cons
All I can say is when you agree to your starting wage, make sure it is as high as you can possibly fight to get, because it is the metric that you will ALWAYS be held against in the event you do receive a raise. Clean Harbors can work you to death (if you want to get ahead), then, if you have an average manager that is afraid of losing their job because they cannot move up fast enough, it stifles the entire group under them. There is also an Eastcoast vs Westcoast conflict (almost like sports), where if you are no in the Eastcoast "club", you will never go very far up the ladder, even though you will hear about promotions all the time, they are all in the east or Canada (promoted from the east). This is a place that you can hang out at forever if you want to give the Harbors your life and get a paycheck in return that does not increase except once every 5-6 years once corporate pays a consultant to come in an figure out why everyone is leaving. . . then you'll get your 3%. Raises are next to impossible to get if you are middle management. General Managers get x% to divvy-out to their entire group and there is not enough for half, let alone everyone, (plus they still need to get theirs, right?) Bonuses, (Maybe), if you are lucky enough to be within a good vertical market segment that actually is growing.