I tried Viagra numerous times, and it works okay, but always gives me a minor headache and upset stomach.  I tried Cialis, and it worked fairly well, but I'll never try it again, due to a SEVERE, long-lasting headache (and I'm ordinarily a headache-free person).  I think Levitra is fantastic, because it keeps me intensely turned-on, results in extended orgasms, and had no side effects whatever for me.  Just one person's observations.