If you'd asked me that question a few years ago, I would have answered, "Maybe." In more recent decisions, courts have been very supportive of these agreements. Our society wants people to be able to rely upon the agreements that they make with each other, and that is expressed through case law.
Having said that, these agreements must be fairly negotiated and fair to both parties. It is worthwhile to have a consultation with counsel, especially when there's a tremendous disparity in the assets that people are bringing into a relationship, and that can occur in first marriages but, more importantly, in marriages later in life in which there are already children of one or both parties who also deserve some protection.
Danny Zack is a partner who practices family law and collaborative law with Kahn Zack Ehrlich Lithwick in Richmond, BC. He has represented the Ministry of Children and Family Development (and its predecessors) in doing child-protection work since 1975. He can be reached at (604) 270-9571. View his firm's Divorce Magazine profile.