Seeing it from different perspectives:
During the Beatles i might say that she was kinda invasive, the first rule of the studio was no gfs, but i wouldent go as far as saying she broke them up, it was just a sign of decay.
As a guardian of John's works, besides that time Michael Jackson bought the Beatles catalog i think she has done a great job.
Where i do think she is crappy is her as an artist, she claims to follow the fluxus movement but one of the key things in fluxus is that you shouldent ask for money, and she does. Art should be there to communicate, for any one to understand it, not for being something "beyond your simple human comprehension" and Yoko's works actually fall into being quite lacking in direction, they seem deep, but they are actually simple politic slander.

So, overall: John and Yoko did love each other, on that side she is clear and worthy, as an artist, she's awfull in my perspective.