I have to put in my pennies worth here, so let me first say that I have never gone there (disclaimer), but know many people that have and here is what I heard:
Don't. I know at least 4 guys that went for a sim prep, and none of them got the job. They were PISSED (understandably, they spent something like 400 bucks on it). They said the sim was nothing like the sim in the interview, and that the test prep was WAY off.
Based on their information, I passed it up (and I did get a job offer in the end).
I would imagine that everyone has a different experience, though.
Good luck all!
B