Stayed in Grand Hyatt Seoul from 23-27 Dec, Club King room. Thought I would add a quick review as last one is few years old. Firstly the area that GH is in is actually one of the oldest and technically the middle of Seoul. It's not in the middle of nowhere as some reviewers have stated, but rather just up a really steep hill that is a pain to walk up. A taxi to Itaewon (main area for foreigner's bars and restaurants) will cost W3000 ($3). There is a local bus that runs up and down the hill, there is a stop right outside the hotel. A short 5 min journey for just W600 will take you to Noksapyeong subway station and from there you can get to just about anywhere in Seoul on the subway. The buses run often. So the hotel is accessible if your prepared to walk a little bit and use public transport.
Hotel itself is 5 star standard with great but over priced restaurants (if you don't have time to visit Seoul), bars are good also but I'd go for the cocktails over the beer as you get better value for money.

Club room is no different in size to regular king room, bathroom is small for hyatt standard (no separate shower). Decor makes the room feel smaller than it is but it's comfortable and practical, however lacks a club room feel. Note that you have to pay for items such as toothbrushes, toothpaste, shaving kits. Again this is not the norm for Hyatt, but they claim that Korean law requires them to charge for disposable items. Hmmmm, funny that local Korean motels provide these items and more, all inclusive. Internet access is not included in club room packages (but I think this is standard Hyatt policy right?) With the abundance of wifi access around Seoul and the fact that local Korean Motels give you internet access and even PCs in your room, I struggle to see how Hyatt can justify charging for internet access for club rooms. Room was clean, serviced everyday, no issues at all with noise/temperature etc... so all good on that side.

The club lounge serves breakfast from 7:00am to 10:00am, It's continental but they do have scrambled eggs. Food is really good, excellent smoked salmon and the wholewheat bread is just like the European kind (i.e. no added sugar) which after you've been in Korea for a while you'll realise is hard to find. The muffins and pastries are awesome also. It can get quite busy (I only went to lounge on 17th floor, there is a lounger for smokers on 15), and staff can be quite rushed to reset tables. The way the lounge is laid out makes you feel like your in a corridor, which in fact you are as it's the middle of the 17th floor with open access to the rooms on that floor and the lifts.... so I'd really wouldn't like to stay on the 17th floor as you'd have to walk through the lounge to get to the lifts every time. Alcohol, softdrinks and snacks are only served during their happy hour (17:30-19:30). Again it can get pretty busy. The snacks are really good though - quick friend scallops, prawns and awesome mini desserts to name a few, the quality more than makes up for the lousy hours. During all other hours the lounge is open but just serves coffee (well you actually have to get it yourself from the machine). I don't know if this normal is Hyatt policy, is it just in Korean Hyatt's where drinks are only served at certain hours?
So basically club room gets your access to the lounge (5 hours in the day though) and dedicated reception.
The lobby can get really really busy, especially on evenings over the weekend.

The spa still had the sign on the desk saying that people with tattoos are not allowed inside, not sure if they still enforce this and I don't think it applies to the pool, showers or gym. Gym and indoor pool are excellent quality, it wasn't busy when I was there.

Unlike some other western hotels in Seoul and hotels in other countries, the Hyatt is a popular night spot and the lobby bar, JJs and most of the restaurants are regularly busy. So there is always a good atmosphere and great service.

The hotel staff are friendly and efficient, because the hotel can be busy they assist you if you need help and approach them rather than them seeking you out.

I've only stayed at one Grand Hyatt before, in Fukuoka, Japan and that was just a standard room, so difficult to compare. However I felt that the room and service were better at Fukuoka and that was just a regular room, not club. So I was expecting more from a club room at the Grand Hyatt Seoul, but it was just not quite grand enough..... It was good and overall I was pleased and happy with my stay. The hotel has a lot of plus points but I think it could be better and maybe other regular Hyatt patrons could comment on what is Hyatt standard or unique to this hotel.