Mine didn't go so well. They flew me out there, then on interview day all of the managers were "busy" so I met with two guys and that was it. 12 hours of flying and two missed days of work for a two hour interview :-(
Ouch. That sucks. I'd think that HR would be very careful about making sure the interviewers are actually available before bringing you out. With mine, there was some concern that it might actually get pushed past Thanksgiving due to lack of interviewer availability (thankfully not, and they got it in front of the hiring committee before the break).
My experience with Google HR in general was very positive, though of course "all's well that ends well", so my perspective might be a bit biased.
Luckily I like traveling and met with a friend out in SF so I guess still a good experience.
After that they referred me to YouTube (is that where they send the second class programmers?) I had one phone interview and he asked me to come up with an algorithm to do something tricky (forget exact question) but it turns out I can't program in my head over the phone. I emailed him the solution later but I guess it didn't help.
