I'm not anti-finasteride - obviously it works well for a good number of people.
But I think Merck have not been entirely honest about the <2% quote of side effects. (Obviously there are a lot of ways to fudge numbers to sound better.)
Anyway, my personal experience is that I took it for a week, after 2 days there were side effects (weaker erections). It's definitely not psychosomatic. I'm not going to go into detail, but you'll just have to take my word for it that I was (and still am) pretty confident, sexually and the side effects just blindsided me. It took me a while to put 2 and 2 together. I thought about just taking it and riding it out, but some of the horror stories of guys in their 30's who are completely finished, sexually put me off.
They say that continuing with the treatment helps and the side effects will go away after a few months. But there are whole communities on the internet of guys who have suffered side effects that have not gone away. That scares the crap out of me.
I want to keep my hair, but I want to keep my sex life even more, and I don't want to risk persisting with finasteride.