The Bernese Mountain dogs I've seen have been very sweet dogs, but I have not known one personally. It is actually a breed my mom was interested in when we were looking to add a dog but I was stuck on collies .

Anyway, I did find this site. Maybe you could ask some questions there, or at least browse through posts with info.

http://www.bernertalk.com/

Some breeders are very willing to educate about their breed as well and tell you all the good and the bad.