Spent a week here at the end of November. To be honest, it wasn't my first choice (Doulos Hotel was booked up).

First the good:
* The room is large (especially by Korean standards). Even the bathroom is huge.
* Very quiet, we never heard any sounds from any other rooms (we were in room 701)
* Ondol heating (under the floor) was quite nice
* Hardwood floors
* Computer in the room with broadband. Annoyingly, it was WinXP with Korean as the default language, so I needed to install Firefox to get English. Unfortunately, there's no wifi, nor could I find any way to get my laptop online (using the cat5 from the computer)
* Complimentary breakfast buffet every morning (7AM-9AM). Consisted of hard boiled eggs, rice, pork products, fried potatoes, miso, creamed soup, mini salad, juice/coffee/tea, toast
* Loads of complimentary items in the room (coffee, tea, bottled water, shampoo, soap, toothbrush, etc)

The not so good:
* The beds were, by far, the hardest, rockiest beds i've ever slept on. I go camping quite often, and sleeping on the ground is literally more comfortable than these beds. They were beyond uncomfortable.
* The location is kinda crappy unless you're planning to do something in the immediate vicinity. Trying to get just about anywhere (by subway) always required transferring at least once to another line, and just about nothing of interest to tourists is within walking distance. Also, the subway/metro directions on the hotel website are horribly out of date. Do not take line 5, or you'll be stuck walking for nearly 30 minutes. Line 9, National Assembly station, exit 1 is a 5 minute walk from the hotel.

Overall, the place was fine. I'd certainly be willing to try it again if there were no other options available at the time.

The wallpaper in the room was getting tatty and peeling away from the wall and the bed was hard (however we slept just fine). Apart from that, everything was good. The room was clean and spacious with hair-dryer, fridge, TV, computer and toiletries (far more than found in popular American motels). The air-conditioning was quiet and worked well. The complimentary buffet breakfast in the hotel's Cafe Mu was substantial and on par with what one would find at Holiday Inn Express or Comfort Inn in the United States. I also was pleased with the location: very close to the National Assembly subway station and the pleasant green areas around the National Assembly building. There is also a Starbucks right across the street and many convenience stores and restaurants nearby. The airport limousine (bus 3060) stops nearby. For less than 100 USD per night, we were very satisfied.

Hotel M was such a great find! We thoroughly enjoyed our stay and would recommend this Hotel to anyone wanting a fantastic base in Seoul.
It is located about 20 mins by subway from the main attractions of Seoul and the nearest subway station is only 2 minutes away. Taxis are also incredibly cheap so if you have bought a few things it is so cheap to hop in a taxi and get a ride back right to the door. Other attractions are close by like the Tallest building in Seoul, Yongsan electronics market and the Fish Market.
The rooms are very big, modern, clean and luxurious. The large bed is very comfortable. The main highlight is the large bathroom with 2 showers, spa bath and a steam room. This is perfect when you have been on your feet all day and want to soothe those muscles.
Breakfast is included and is served on the 2nd floor restaurant. The food is fairly good and changes slightly every day.
Free internet and use of a computer in your room is a bonus.
This hotel is extremely good value for money. We were able to secure this room for around $65 (65000won) per night through the Innostel website. This is part of the visit korea year special.
Hotel M is the perfect base for anyone wanting to explore the many sights of Seoul.