I personally think that my Cervelo handles curves very well...almost as well as my roadie. With reference to downhills, I think that's pretty ridiculous. I've gone 45+ mph and felt rock solid.

I would say the biggest reason not to buy a tri bike is if you are planning to ride with people most of the time. The only time I ride my tri bike is when I am by myself because it can be unsafe to ride with others when you are in the aero position.