The interview part day is ok, there's a psycho test of just over 200 questions, a verbal reasoning test and a numerical test. Interview is then fairly informal standard interview, there were no tech questions or gotchas. It was very friendly and relaxed throughout.
The sim was quite complex (in my humble opinion). It was in the 737 300 and consisted of take off and climb to 5000', level off and accelorate to 250 kts. Steep turns, one at 45 degrees bank and one at 60 degrees, then descend and level off. Engine failure to deal with before vectors to single eng ILS, go around to 3000', the engine is then restarted and further vectors to 2 eng ILS to land. All completed on raw data, no fligh director or automatics.
It was hard work in the sim, but very enjoyable. No knowledge is expected (except of flying of course). There's a very thorough briefing beforehand. They're just looking for your ability in general, not 737 knowledge.
Main thing is to relax and enjoy it, easy said, but it worked for me.
Good luck.
Jonathan