I have all three and have taken different ones at different time...not unlike the other gear we use.
My findings:
Viagra: Gives me a headache. Works well. Onset usually in 20 minutes. Lasts about 2 hours. Seems to be slightly diminished with alcohol, but REALLY diminished with any fatty foods. If you are having dinner and drinks first, forget it. Probably my first choice when I know something is going to happy, know when it is going to happen, and know it isn't going to last all night.
Levitra: Very, very similar to viagra. No headaches. More sensitive to alcohol than Viagra, but much less sensitive to foods. Erections are about the same as Viagra. Onset and duration are about the same. My first choice if I'm going to dinner first.
Cialis: If you think of Viagra and Levitra as a mountain peek, Cialis is a rolling hill. It doesn't have the immediate and intensive impact that Viagra and Levitra have, but it does last for two days. No headaches, not sensitive to foods, slightly sensitive to alcohol. Great for a weekend when you don't know what will happen or when.
Depending on what I am doing, I may take any one of the three or mix them. In my Lifestyle (*) I've found that Cialis as a base followed by a viagra or levitra about 1/2 an hour before festivities results in about 4-5 hours of multiple sessions.
As Mauit pointed out, all three are vasodialators, i.e. they make it possible for more blood to flow into your penis. You have to be turned on and want to have sex before anything happens. But if you are turned on, you'll be ready to go. Note: because they are vasodialators, mixing them is not recommended. On the other hand, the AMA and FDA studies I read overdosed subjects to 10 times the recommended dosage with minimal and somewhat superficial side effects. I figure that I am in good shape and that taking one cialis and one viagra in a night is not going to damage me...but if you have circulatory or blood pressue problems, you should probably stay away from them.
As a side note, even with Viagra and Levitra that are supposed to only last a few hours, I always find myself hard and wanting it the next morning...but maybe that's just because I'm a male slut.