Postdoc here at Stanford (Did my phd in Ivy). I agree with everyone that the advisor is the most important thing, but it's very hard to judge his/her character before joining the group, no matter how much you talk to people. Sometimes the advisor is very nice and his/her working style just doesn't match with yours. Also you kinda have to like your lab partner to have a good phd experience (I was extremely lucky with that). I like the Stanford's rotation system as you can try out different labs before joining (don't know if Berkeley does the same thing)