At the beginning of the year I went through a series (5 or 6) of technical tests for a huge company (got the job, turned it down for a number of reasons) - a couple I thought I'd fluffed terribly, but it turns out the interviewer just had that demeanor in interviews and it was standard practise to push the interviewee until they couldn't answer any more - both to find their boundaries, and to see how they deal when they don't know something.
I was totally worth the preparation though (basics, problem solving in general) - in fact that's probably the best way to approach these things, as a huge game that you're trying to get as far through as possible.