I went to Berkeley for an on-campus interview and was very impressed.
Berkeley has a very prestigious academic reputation. I interviewed with several professors there. I was very impressed by their passion. Many of their work was really groundbreaking cutting edge. Also I found a few professors whose projects interested me.
I also really liked the easy-going atmosphere at Berkeley (the so-called "Berkeley casual"). The people there were all very friendly. The relation between different labs in the MCB Department was very open and cooperative.
And there's also the irresistible climate factor. I went to Berkeley in early February, but it felt like spring. There was no snow, quite surprising for a place with such high latitude. The campus is also really beautiful, especially that there is a view of the Golden Gate Bridge from the department building.
Compared with the other offers I received, considering academic prestige, atmosphere, climate and environment, Berkeley was quite irresistible to me.