Its pretty horrible. Management sucks. The unions are wishy washy at best. Pay is great if you want to work 7 12s during the holidays and 6 12s before you go regular. Depending on the craft, it could be 1 to 12 years before you get a regular fulltime position.
If you like being on the move & can handle continual lifting of packages of varying weights it's a great job, but if you have any difficulty with lifting, or being on your feet 8 + hours a day at times, it's not the best fit.