Pros
McMaster-Carr pays very well and the benefits are absolutely outstanding...better than any company I've heard of by a long shot. Even the guy who separates a box of 1000 screws into 10 boxes of 100 screws is paid well....and he has a bachelor's degree. At other companies, a forklift driver probably barely has a high school education and may be considered to have a lowly position, but at McMaster, he might have an MBA (they offer tuition reimbursement); at McMaster, the forklift guy started out doing something simpler (like counting screws) and worked his way up to driving a forklift.
Cons
A little background: There are basically two employee tracts at McMaster-Carr; there's the employee track and the management track. Very, very few people make the jump from the employee track the management track. A large portion of those on the management track, or MDs (management development) are very smart people who came out of high quality colleges, but that does not mean that McMaster will not chew them up and spit them out. Most MDs are less than 5 years out of college. There is a high turnover rate among MDs It's a very intense work environment for people in management; the people at the very top have no problem completely humiliating management in front of large groups of people. They hold management to very high standards, and sometimes those standards are a moving target. You can never know what seemingly minor offense they'll let slide one day that they'll fire someone over the next day. Management works long hours (60 hours is standard) but a benefit of being a regular employee is that they do not work long hours. So for management, the work life balance is atrocious, but for regular employees, it's great. If you work in management at McMaster, you will most likely eventually go on to a different company where you'll be able to use a lot of what you learned at McMaster to be a better performer...and you'll also be glad to be free of McMaster. Note that the Elmhurst location (headquarters) is much more hardcore than the other branches. If you're a regular employee and you show up to work on time, work hard, and do your job well, you'll be there for decades. But if you're in management, doing your job well doesn't mean a whole lot, which fosters an environment of fear.