Frankly, it's been the best hourly return on investment of anything I've done in my life up to this point, by far. Assuming I wouldn't have gotten the job otherwise (which seems reasonable), each of those hours spent studying has proven to be worth several tens of thousands of dollars. I'm not exaggerating; I just did the math.

Maybe the interviewing process is broken or sub-optimal or whatever, but it is what it is, and if you can get through it by doing some additional studying, then it's absolutely worth it. Google is a good place to work on designing big systems, so if that's your interest, consider just putting in the work.