On the other hand, I just read my Cancer profile on astrology.com, and it's pretty damn close, even down to my technological preferences when it comes to computers (mundane programs, web design...screw the flashy pretty stuff. If it does what I need it to without the flashy crap, all the better).
I think one thing people forget though is that astrology doesn't make the person. We make ourselves. In the case of the two twins, how many twins do you know that want to be exactly alike? I don't know very many. Most that I've met (which I'll admit is only maybe 5 pairs) want to be as different as they can be. If you take two Cancers, put one in an abusive home, put the other in a loving, nurturing home, they're bound to turn out different, to some degree.
As for the generality theory, what if I told you that you can be highly manipulative and are prone to brood and even be downright nasty when you don't get your way? Would you agree with me? Those are qualities of Cancer, and I have to admit, I've found them to be true in myself.
Personally, I do think that astrology has some credibility. Sure, it's not gonna describe you 100%. For example, mine says I shouldn't like to travel, but I have a love of travel, and intent to travel all over the place once I'm able.
A curious theory I've developed is that Water signs may be a little more inclined to poetry and other works of literature than other signs. Anyone care to put it to the test? How many Cancer, Scorpio, and Pisces signs do we have?