To get free shipping you need to order $30 (if I recall correctly) of product each time.   With Prime I can order a package of watch batteries or a book that is only $10 without having to add on other items or paying shipping costs.

Note that Prime also provides access to online movie and TV content which up until recently hasn't been a big deal for Canadians but Amazon clearly intends to challenge the dominance of Netflix.

Obviously it comes down to your individual shopping habits but I would say that anyone who shops on Amazon 2x a month or more likely would benefit.