My girlfriend lives in Pohang and I spent a summer there last year, visited a lot, and will be there this summer again...

Bad news first - it's kinda boring. Especially during winter. The 'downtown' area is kinda small and there aren't a whole lot of fun nightlife places. There is a US Marine base nearby, so you see a bunch of marines walking around making trouble sometimes (nearly been in a fight or two). Also, bus and train tickets are a little bit pricey out of the area (but local buses and taxis are dirt cheap).

Some good news, the beach area is pretty nice, and it is a lot of fun to go with friends and have some fresh seafood at any of the quaint restaurants along the area. Daegu is pretty close, and the nightlife in Daegu is pretty awesome if you go to the right places. You are also really close to a lot of beautiful old cities like Gyeongju (The place is fricken beautiful, you have to go). And the people of Pohang tend to be really friendly and easygoing.

Ultimately, the trip will be what you make of it. Unless your school has draconian rules that prevent you from going out, drinking, and having fun, you should be able to have a really fun time no matter where you go in Korea. And the country is so small, you can take weekend trips literally anywhere.