I started taking Viagra during a period of high stress and depression to help get things going. Emotional distress makes it difficult for me to maintain a good erection, so it was a miracle worker for me. Now I take it mostly recreationally as I am on anti-depressants and a low dose blood pressure med.
Personally, I love the extra power and oomf it gives me. Until the period of depression, I never had any issues, but when I'm on it, I get the most rock hard, engorged, powerful erections ever, and a significant stamina boost.
I will say that occasionally I get the headache and/or blue tinted vision, especially on an empty stomach. Also, to confirm others, yes, drinking or eating heavily before you take it is going to reduce effectiveness. Always give yourself 30 min or so at minimum for it to kick in, no matter what. An hour of you've eaten/drank alcohol. Your body needs a minute to get it into your system. Only other thing I would caution is that you will achieve a fucking great erection at the cost of some sensitivity. Not a bad tradeoff, as that DOES promote stamina. Just don't expect to cum quickly unless you are extremely stimulated already. Viagra isn't designed for quickies, as I have discovered firsthand. Great if you want a night of long, amazing fucking, not so great if you are aiming for a morning quickie or whatever before you have to be somewhere.
Other than that, dosage is a bit of a crapshoot. Everyone is different and will find different dosage effective. My doctor prescribes me the 100mg so I can break the tablets in half (cost of the prescription is the same no matter what dosage). I find that 50mg is ideal for me, 100mg is good if I really want to be able to go for hours with multiple sessions. You can also stagger doses, but don't exceed 100mg per day. I like to take two 50mg halves a day when I know I'm planning on a few fuck sessions. But again, everyone is different, and your needs and sensitivity will vary.
Other than those cautions, I honestly really recommend it, for need or just fun. I've had really good experiences so far after a couple of months of need and recreational use. Just relax, don't overthink it, and have fun.