As one of the previous writers noted, the requirements for PBK vary from school to school. Some just invite the top whatever percent to join. Others have different criteria. The only absolute criteria that I know of is that you must be in a liberal arts program: NOT business, NOT engineering... The national PBK website mentions foreign language a math and foreign language, but I had no college language. It also talks about being well rounded. They have guidelines, but its up to the school.
At my school, the individual departments selected who was in PBK. This balanced out difficulty differences among majors. For example, you could have a pretty "low" GPA (3.5) if you were a math major and still get in, but if you were a Psych major you needed to be smoking good!!!
As for whether you join. I will say unquestionably yes. I was inducted about 10 years ago and it has consistanty been noticed by people looking at my resume. No one seems to care about dean's list (not even on the resume any more), freshman honor societies, honors thesis or even class rank. I think people use it as a surrogate for class rank. They know that only a small percentage make it each year. Its kind of like AOA in that regard. In the real world very few people include their GPA or Class rank on their resume that looks pretentious, for some reason listing PBK is not seen that way.
A word about the resume too. I don't know much about the "medical CV" as a form of resume, but I can tell you about a regular resume. As you move on in life, you have less and less room on your resume, education becomes secondary to experience and you must decide what to keep and what not. On my resume, the only thing I have is my college, majors, title of my honors thesis and the level of honors I revieved (i.e., with honors, high honors, highest honors) and PBK. I only included the thesis because it was relevant to my profession! Unfortunately, I cannot list the rest of my stuff, there just isn't room and no one cares that I was in the orchestra or a freshman honor society.
Ed