It is not 100% accurate and it can take months to get through the process, but if we do it right we will be spending years together. So, it is worth the investment in time.
Don
However, the real problem is the amount of bias they introduce into the process. Few Google products have a lot of polish and they really don't get the social networking side of the web. Conceder how much effort it would take for a standalone version of gmail that could easily replace outlook everywhere and make Google a ridicules amount of money.
Google should be doing better than that -- much better.
I think there is a real danger here of extrapolating the experiences of one or a few people to a statement on hundreds or thousands of interviews/interviewers. At the rate Google is interviewing candidates it should be expected that some interviewers are not good interviewers.
After doing a lot of research on the subject (to prepare for my own experience) I can say with some certainty that based on reports from people that actually went through the experience, interviewers asking for brainteasers or "random knowledge" are in the clear minority.
(At least for software engineering interviewing. The questions for product managers seem to be a whole other beast.)