Hey there, this post caught my eye in passing since I'm currently taking UCBX classes right now (Med Vir, Med Microbio, and Biochem).
In terms of difficulty, if you have a decent biology background, I doubt you'll have much trouble if you put in *some* effort studying. Since I'm taking all night classes, I get the distinct impression that the teachers really simplify the material so that the course isn't too difficult. Example: Biochemistry with Prof. Yund is open book, open notes. The cut-off for an A is 85%. Med. Virology allows you to drop two of your lowest quiz scores. Med Micro prof always points out what she is going to put on the exams, etc, etc.
So far, the professors don't strike me as being especially personable. It's hard to develop a relationship unless you make a big effort or see them in lab. But they are nice and approachable. Lectures can be tiring since the professors are tired and the classes themselves are way too long, sometimes torturous for both parties if the material is not very interesting. But if you're under the mentality that you gotta do what you gotta do to get ahead, it's not so bad.