I stayed in Hongdae Guesthouse with a group of 5 friends in Feburary and I would highly recommend anyone to stay here. Mary and her staff provide excellent customer service and was willing to help us out whenever we needed. Even gave us free samples of her food :)
The room we had was great had every facilities we needed, warm showers, washing machine, free internet, free water (dont need to go 7-11 to buy water) and beds were comfortable. Location is excellent aswell with food stores easily accessible. etc Paris Bagette and Dunkin Donut.
Will definately come back when I get the chance and hopefully explore S. Korea a bit more. I miss Seoul