Recently stayed at the Yoido hotel - was booked as part of a Seoul tour.
The hotel itself is, as my title suggests, perfectly fine, though nothing especially exceptional. The lobby is cute and nicely designed, with a small reception area, a coffee/tea/buffet area where they do light breakfasts, and a small "business center" with computers available for guest use. There's also a small gym that I didn't see and can't comment on.
The rooms are medium sized - not too small by Asian standards. They're pretty basically appointed and feel somewhat worn/dated all around (furniture, linens, fixtures, etc). But it seemed clean and safe.
Had a problem with the WC and the staff came up immediately and fixed it, so kudos for responsiveness.
Location depends on your plans. Its right in the Yoido area, not too far from the LG towers and about a 15 minute walk from the nearest subway station. Getting to other parts of town via taxi can be expensive, so unless you're planning to be in this part of town for a reason, you may want to consider other areas.
At around $100/night, its relatively well-priced by Seoul standards. Basically a 2-3 star hotel, that's clean, safe and had good service.