I used it once, was hard for 2 hours (not the best idea when you are 10" and just trying it to see what happens), and it made my heart feel like I was on 100 cups of coffee. Never tried it again
Little known fact: Viagra was accidentally created when they were trying to make an unrelated heart medication. It's been shown to interact with many other medications and substances in such a way that it can give you a heart attack. You've been warned.
My opinion: If you're deadset on taking it, start with the lowest dose available (25mg I believe) to see how it affects you. If you want to be even safer (the safest solution is not taking it at all, but...) then ask Pfizer for an email of the relevant safety information (known drug interactions, etc.). You could also be up front with a doctor and just tell them straight up "Look, I'm thinking of taking Viagra recreationally. I know you can't assist me in acquiring it if I don't actually need it, and I wouldn't ask you to put yourself at risk doing so. I was just wondering if you could give me any tips or warnings about using it so I don't accidentally, you know, kill myself with a surprise boner through the forehead or something." If you make it clear that you're going to use it and have access, they'll probably tell you the information in a heartbeat. Some may even write you a prescription for the smallest dose, just to make sure you're not getting a dangerous counterfeit drug. Either way, most doctors will try to help you if you're seeking information to protect yourself from harm.