I think it is beneficial when it helps people become comfortable with the idea of sexuality, and also become more comfortable with and aware of their own sexuality, get a better idea of what they might like, what they might not like, etc.
I think pornography is harmful when people base their ideas of sexuality off of it and develop unrealistic ideas about sex, the body, relationships, etc, and when it degrades the importance if intimacy in a relationship. Porno is a form of entertainment, it is not a guidebook that will teach you better ways to have sex, how real people have sex, and it ignores the potential consequences of sex.
So, as with all things, there is a good side and a bad side, and it all depends on your personal perspective. I think this is one of the main reasons that we should have age restrictions on porn, and also why sex ed should be taught in school. If you have the context to view it properly, then it's great and harmless fun, if you don't have that maturity and knowledge then it can be damaging.