"Park Avenue Guest House is a charming find "
I stayed here for 8 days in June, and the experience exceeded my expectations. Park Avenue is not a full service hotel, but it is an affordable, friendly, and convenient alternative. (For example, there is not daily housekeeping--empty your own little trash cans in the hallway trash cans, and trade in your used towels for clean ones at the front desk.) The best part is the staff. Everyone was so helpful and friendly, from the housekeeping staff, to the managers, and especially Sylvia at the front desk. Sylvia speaks fluent English, and went the extra step to help us with directions, phone calls, looking things up online, etc. Most everyone speaks some English, so they are able to communicate with most non-Korean guests. If you want to be in the Hongdae neighborhood but not spend a lot on lodging, choose Park Avenue!
The rooms are pretty small and compact. We had a room with two twins, and there was barely enough room to store a couple pieces of baggage. The bathroom is pretty small as well. Still, everything you need is there--air conditioning, TV, small fridge, wifi--and besides, who spends a lot of tie in the room? I think most of the rooms are the same size, so if you need a room with two beds, I would definitely consider only twins. It would be pretty crowded with a double and a twin, I think.