Namu is a great place to stay - the guest-house is in a quiet neighbourhood surrounded with places to eat or grab a cup of coffee on your way to the metro station (which is not very far). On the other hand, Hongdae is very nice and interesting both during the day and at night so you don't need to go far in order to have a good time.
Also, the managers are wonderful - always ready with advice and very helpful if you need anything. Helped us a lot with instructions how to get to places we wanted to find, never tired of us asking what is this stuff we have just bought and even pointed us to the best Patbingsu :) Really made our stay more pleasant!
Finally, the place itself is very clean and cosy, offering many services (the washing machine being an added bonus as we stayed for more than 16 days in Korea and didn't want to carry too much luggage). Felt really safe and welcome there!
Only downside from my point of view is there are only 2 shared bathrooms and when the guest-house is full, you might need to wait to get to use them. Nevertheless, really great place to stay in Seoul!