An overdue review! Stayed at the Tea Tree Hotel for seven nights with friends in March and loved our time in Seoul. The hotel is a good choice if you don't want to splurge too much on accommodation. We had a twin room for less than USD$150, very decent as hotel rates in Seoul tend to be rather pricey.
The good: The hotel is new and on a very charming street, with lovely cafes and little boutique shops. Tons of cool, chic-looking locals hanging out at nights and on weekends, good for people watching. Loved the vibe. The subway station is walkable distance and I like that the hotel is homely and quaint, not one of the typical city types.
The bad: The furnishing of the hotel room is on the cheap side and I doubt they will wear well. No storage space whatsoever and even the bedside drawers are decorative and cannot be opened (???). No separation for the bathing area in the washroom so the entire floor gets wet. Some of the staff are not too fluent in English so we had some problem communicating with them when the lid of our air-conditioner fell out or when we wanted to ask for directions.
I'm definitely going back to Seoul, but will probably try out another hotel in a more central location the next time. However, Garosugil is such a charming street that I'll still recommend Tea Tree Hotel if you don't mind a simple hotel with some rough edges.