Just from my experience, the generic Sildenafil doesn't work as well as the brand name. Problem is that most drug-plans won't provide the name brand and automatically substitute the generic EVEN WHEN the doctor states not to do so. Also, side effects are different on everyone, so don't take more than the prescribed dose.
Hopefully the Canadian sort are the real deal, as the sites from India and the islands (Bahamas, etc) are usually fake and there just to get your credit card info.