Pros
Custom heavy trucks are very challenging engineering subjects. With each truck being essentially unique, some very complicated engineering challenges emerge, and these are very satisfying to work on and solve (sometimes). The huge inertia of the company can be frustratiing, but it also makes it rather easy to stand out, to introduce new engineering techniques, and to pioneer new techlogies, which can be satisfying and a good resume padder. You can get a more varied work experience at PACCAR than just about anywhere, if you are persistant. Also, they have lots of money, and sometimes you can get them to spend it on engineering tools.
Cons
Management rules by fear. Each level of management is trained to be afraid of the next level, so most of everybody's time is spent making sure that nothing happens that will displease the next level of management. The CEO rules with snide comments and caustic remarks such as you might have heard from an English lord in the 1800's. Way too much emphasis is put on proper dress, such that you could be a complete idiot who dressed nicely, and get prmoted time after time. Agree with the executives and always wear a suit, and you don't need a functional brain. Finally, PACCAR has no new product developement capability at all. A new truck for PACCAR is moving the air filters inside the hood and streamlining the fenders and bumper. They are successful pumping out conservative products to a conservative market, and problems tend to get evolved out of the product over the 30 year life of each product. Don't go there if you like to innovate.