Here is my LTP perspective based on a recent stay over 3 nights:
-Overall service is attentive and English language skills amongst the staff are strong. I will say, however, that problem solving little issues is similar to other properties in Asia and can be rather tedious - this is a cultural issue that one who travels throughout Asia should appreciate and not get too offended or upset about. I will also note that I appreciated the meticulous servicing of my room by the housekeeping staff.
-The physical properties of the hotel are great - room furnishings etc. The views are spectacular.
-The location is actually quite convenient - above a sophisticated shopping mall with really great food options (I have an 'in' with Korean food and will say the quality of the offerings and variety at D-Cube city is great).
-The subway/transit system literally just steps away. One stop on the train and you are at another similar, albeit more extensive mall - Times Square (have stayed there at the Marriott Courtyard as well). Getting to Central Seoul or elsewhere is pretty manageable via the trains or even the reasonable cabs.
-The club lounge had an extensive breakfast as well as one of the better spreads amongst SPG properties in Asia for happy hour, including self-pour booze. The views, again, are quite nice. And, unlike some properties (particularly the Westin Nagoya Castle) if you want to take a bottle of water or a small can of club soda with you as you leave the lounge, there is no issue or shame. I did notice on one night a small group of girls who were happily hoarding numerous cans of Heineken to take with them...
-As a LTP I took advantage of 2 breakfasts at the main restaurant - Feast. The buffet was extensive and vast (too much really), the quality quite good and the service particularly excellent and attentive.
-The exercise facilities are excellent, the equipment new and clean (as well as lots of it) and the Sauna was self-service at no additional charge. Towels, robes, exercise attire are all readily available.
-The elevator changing can, as others have mentioned, be slightly tedious but I didn't mind changing elevators as much as the time it often took for an elevator to arrive (particular the elevators to the ground floor and mall). This is a minor issue for me however.
This is my new favorite SPG property in Seoul (why on earth stay at Walkerhill - so inconvenient for public transportation etc?).