Pros
Biggest pro in my opinion is super flexible hours. Good people to work with (with exceptions of course). Good benefits. Great place to gain experience (especially coming out of college). Job security. Overall: Not a bad place to work, but there is definitely better opportunities out there. Don't expect big money. Most employees use this as a stepping stone.
Cons
$$$$. In my experience the reason most people leave is because they don't get paid enough. Majority of the work force is straight out of college so they have school loans, car payments, rent, etc. This leaves little in regards to disposable income making it hard for most to save money or get out of debt in a timely fashion. VERY high amount of turnover which management doesn't seem to mind. Why pay someone more money when you can just replace them with the next college grad? I've heard the term "White collar sweat shop" get thrown around a few times. Being that it is a contract lab, at times the work load can be demanding. It's also a very regulated environment and mistakes are tracked very closely and held against you. I've seen plenty of senior/principal chemists get hired from outside pharma companies only to get demoted/fired/quit because the environment is not as laid back as some of your big pharma companies. As with most large companies, there is a disconnect between upper management and employess who actually do lab work. At times this can be quite frustrating when decisions are made that make it more difficult to do your job. (All of these experiences can vary depending on who your immediate supervisor is/what department you are in, but you will not escape them entirely even if you land in the most ideal position.)