Pros
I've been contract and full time with other big companies (Google, IBM) and Twitter has been the most open and hands on, fair and gives you the most overall ownership of your work. Definitely competitive in wanting to get the job done- but we work as a team together, never against each other. I really appreciate how hands on and available the hiring managers are- they want to hire great people on their team and are completely accessible to us so we can do this! The team is fast paced and dynamic, you will grow while working here.
Cons
You are expected to hit the ground running and know how to do the job you were hired for, which is fair of course. Training is supplemental, but you can't expect to come in and learn how to be a sourcer/recruiter, they are too fast paced to hold your hand. I don't think this is a con, but it's good to know if you're not very experienced.