Good news: It's pretty much all benefits.
Masturbation. The beast with one back. The most popular hobby on Earth. The five-minute staycation. Almost everyone does it, yet many people still haven't gotten the message that it has lots of health benefits. The only downside is that your personal routine can become... routine. Your body gets used to it, so vary your method of manual stimulation to keep your genitals flexible, fun, and ready to do new things with your significant other (if you have one... and if not, variety is still fun) when they're not also masturbating.