I just got back from Seoul where I stayed at the Millennium. This thread was very helpful in making my selection so wanted to provide some feedback to help others.
The Millennium was definately the best choice as far as location is concerned. Being within walking distance of Seoul Station put us in the middle of all our excursions. When I saw later where the Grand and the Conrad are located, this was the best place to be.
The room was great. We had the hill view and the room did not seem too old at all, although I did discover a leak in the sink drain pipe but a call downstairs resulted in a maintenance guy showing up within 3 minutes to fix! We left to go out and when we came back it was all taped up and did not leak again. The room control panel was cute but did have some trouble getting the lighting to set as we would prefer despite the vast options.
We got "upgraded" to a room on the executive floor. Nothing special, but plenty big enough. I don't know if this hotel has any suites. None were coming up for booking in the week leading up to our stay but maybe were full?
We took extensive advantage of the executive lounges. There are two, each identical, on the 19th and 21st floors. The breakfast was great with mostly american food, but a little Asian. The salmon was always there (both smoked and teriyaki) as well as the Swiss Muesli.
The evening happy hour was also great with enough hot appetizers to make a meal of it if you wish. Unlimited free beer, wine, champagne, and certain liquors! Ask for fruit juice if you need it and they will bring promptly! I was surprised when at other Hiltons I had found the alcohol to be an added charge.
On checkout, we had no charges despite the internet service prompting us to pay every day. The porters were very active in collecting our luggage and bringing it up when we arrived but we brought our own bags down when we left since we were in a hurry.
A note about getting up and down the hill from the station. There are a couple of back ways in to the hotel avoiding most of the hill climb. If it's during business days/hours you can go into the Daewoo building (from Seoul station go to the street to the LEFT of Seoul Square and go up about a block or so, on the right) and take the elevator to the 8th floor. Immediately to the right of the elevator is an umarked door. Open it, and find a bridge to the Hilton garage. Then go across the garage to unmarked doors (TWICE!), then will come into the elevator lobby on P6. You can walk up one flight of stairs to the main level and then take the elevator up to your room. This is not intuitive but once you figure it out, a great shortcut.
Otherwise, pass the Daewoo building, walk up the driveway following the Casino/Hilton parking signs, go into the garage where the cars enter and go to the parking elevator and take it up to the lobby level. Walk through the hotel to the front desk or the room elevators.
For eating, I recommend going right across the street from the Hilton entrance to a strip of mom and pop Korean restaurants. Our favorite is on the 2nd floor labelled "Korean Restaurant". Just walk around the the left to enter from street level onto the 2nd floor. A few great oyster dishes. One you can get about 18-24 pan fried oysters for 12,000 won or about $11! You probably need to share with someone! We ended up eating there 3 times.
We did explore the area where the new Conrad has been built. They are still doing final finishing work so it is definately not ready although the surrounding International Financial Centre is now open. The Conrad is in a business area of Seoul but down by the river and parks, so lots of great walking. It seems though that the shops and restaurants would be quite a walk away, although some western food is available in the arcades/malls around there during business hours. Not the greatest spot for tourism but will be interesting to hear how the amentities turn out.