The guy I know who takes it swears that its a miracle from God himself. And seeing the pictures of his father and uncles at his same age, I tend to agree.
Seems to be important to start treatment as soon as possible. The more delay, the more irreversible the loss becomes. For some people, this may very well be in one's early 20s. Not exactly a time when young men feel like going to the doctor and taking pills, but its a lot cheaper/easier/safer than regretting it later.
I think when he started, a prescription for the medication was $300-$400/year, and he obtained a concentrated super-strength version, cutting the pills up to the dose for hair loss. But prices have come down when the medication went off-patent. Treatment costs him all of $30-$35/year now.
Only one big side effect, and that is, do not let the fertile females in your life anywhere near the stuff. The birth defects can be severe if they are exposed, even inadvertently to it in pregnancy.