I for one can vouch for the ATC prep. I did there Alaska Airlines prep and thought it was excellent. I could not have passed the sim ride if it wasnt for my training at ATC. There written gouge was pretty old but fairly on track.
The big item was the sim prep. It has been years since I had flown arcs and intercepted radials outbound off of the arc, ect.
The AK sim ride went very well due to there training.............
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.