The breed standard for Berners is confident, alert, good natured and never sharp or shy and aloof to strangers. My last Berner, a male, was a mellow easy going boy who was aloof to strangers. My current Berner, a young 1.5 year old female, couldn't be more different. She is extremely athletic, has a very high metabolism and requires a lot of free exercise and socialization with other dogs. She is not aloof at all. The males are more aloof than females, and males can often be more mellow in adulthood, although there is no guarantee of this.
Berners, if from a reputable breeder and not a puppymill or backyard breeder, can be relied upon to be highly dog social and like other dogs. The important thing to remember is there is no breed guarantee, so you must evaluate the parents and their temperaments and energy levels. Most young Berners, if healthy, are high energy until 2-3 years of age when they mature and settle down. A highly energetic young Berner will likely be a problem for small dogs but will ultimately learn not to squash them. If you are looking for mellow from the start, there are better choices to consider.
An important aspect of the Berner to understand is that they are highly touch sensitive and emotional. This is in stark contrast to breeds who are not touch sensitive at all such as Labs and GSDs. Berners are highly sensitive and easily damaged by harsh treatment and aversive training methods. We have seen many in this breed ruined by pinch collars, harsh verbal corrections, and newspaper swatting.