I had a really hard time feeling like part of the community during my first month. No one came over to ask how I was doing. No one held my hand. I didn't get many newbie lunch invites. People look so busy, or tight-knit, that I had to go against my natural impulses and really try to meet people. I'd like to see a better onboarding program in the future.
Everyone needs to work long hours sometimes. Some people work long hours all the time. You don't have to be a martyr. As one of my interviewers told me: be committed, but have boundaries.
People seemed surprised that I didn't know how a completely proprietary information system worked yet! People will help you, but you need to ask. It's OK - just don't ask the same questions over and over again.
There are some engineers who come off as jerks, more than at other places I've worked. They're really not, but just be prepared, be nice and don't expect everyone to be everyone's favorite right away. There are a lot of exceptional people here... some of them are just more "special" than others :)
Much of work feels unstructured. If this is frustrating, DON'T WORK HERE. If this is exciting to you, and you can deal with that or help create some good structure, WELCOME TO SPACEX!