We chose this place as it's kind of hard to get a access to a 'real' onsen (water from natural hot spring, not just 'hot water' like a Sento). The other Dormy Inns in Tokyo don't have natural hot spring (aki may be mixed water or something).
Plus - natural onsen, large buffet breakfast (it may or may not be included in the price - it's worth Y1100), rooms with windows that open, close to metro, free shuttle to Tokyo Station, close proximity to Tsukiji, super fast LAN internet, 2 computers at lobby with internet, free ramen from 9.30 - 11, friendly staff (they have English speaking staff too for those who need it), free morning coffee, nice fluffy towels, relatively big rooms by Japanese standard.
Minus - location is a bit out of the way (even if it's just a few stops from Tokyo Station) and not really walking distance to night life so you're going to rely on taxi if you stay out after the metro has closed (also not many 'breakfast' options as other areas may have, you've only got 1 Lawsons nearby), the lifts are super slow, slightly dated decor (which makes the room look smaller too), paid wifi (we have our own wireless router so we don't have to pay), better booking bargains on the Japanese only website if you can understand Japanese booking systems (and if you book fast enough!) otherwise the prices on the English speaking sites are higher (but have more availability of rooms), no extra discount if you chose to be 'eco' friendly (like other Japanese hotels), onsen can get very busy in peak hours.
I gather they cater to salary men so they've gone out of their way to make sure there are lots of home options like the free ramen, shuttle bus, hearty breakfast etc.