I did about 20+ technical interviews at Google/Facebook over the last 2 years.

Overall, the interviewers were competent. Sometimes you could tell that they didn't really enjoy interviewing people but it was ok. I only had one interviewer I'd consider bad (gave confused advice to lead me to his solution), but maybe it was just a personality mismatch.

I had better experience with more senior interviewers too, and I found Facebook better than Google (although similar), on every respects (choice of questions, team matching process, more interesting projects...).

My main complaint is that I've found the whole process random. Right now I'm in the team matching phase at Google, but not sure it'll go through (and it's not the best time).