I have been with a polar opposite person before, and while it was fun and interesting in the beginning, our differences started becoming very apparent. In the end, it almost turned into an emotional tug-o-war, of sorts.
My advice would be; if you are looking to have fun on a temporary basis, then go for it, but if you are looking for a long-term relationship, you might want to find someone else.
The best relationships are usually between people that compliment each other, rather than pull you in opposite directions. Like Stargazer43 said, sometimes opposites can compliment each other, depending on what the issue is, though.