I have never practised Hapkido, but I know of someone who trained in this system in Korea, along with it's sister martial art Sulkido.
Hapkido is a system similar in philosophy to the Japanese Aikido, following the circle and point techniques also practiced in Jujitsu.
It is very effective as it has a diverse syllabus of striking, throwing and grappling.
Having never seen it, I know nothing of it's power in reality, but I once knew a Korean History student who had a brother in the Soul police force. They use a Hapkido style system for self defence and law enforcement, so it must be good!