Pros
Most people are great. No one on the ground treats contractors any different than 3Mers. Culture varies a lot by group (it's a huge company after all). My group was great, very relaxed and friendly atmosphere.
Cons
Hopefully you got a job that interests you because there is little to no chance at advancement as a contract employee. Your manager will probably talk to you about a conversion, but their plans will be subject to 4+ managers and executives above them. Every time one of those positions changes, all plans get reevaluated (and very often thrown out the window). Since most management positions change every 2-5 years you are pretty much guaranteed to be chasing the full-time conversion for 3-6 years, but really it is never guaranteed and I promise you they will try to low-ball you on the offer (make sure you take those years to learn the pay scales and pay attention to min/max/market). Honestly, I had to have had one of the best contracts around (20 hrs per week, I set the hours, pay rate comparable to T3 engineer) but even I grew tired of it after 2 years and got a much better full-time job elsewhere.