We stayed at Citizen M for a couple of nights in November 2012. It is very new and, you sense, still finding its feet.
The good things: This is a very friendly hotel. The staff are attentive and helpful and they really convey an enthusiasm for their hotel that is infectious. The slightly Japanese design to everything is very attractive and the overall impact - of the lobby and the bar area - is dramatic and pleasing.
This hotel is very clean - in all areas - but that is hardly surpising in so new a building.
The bad things: The rooms (based on ours - 321) are very very small. The bed is large, which is nice but no room on either side to get in an out. Nowhere to place a suitcase, other than on the floor, and certainly no chance of swinging any cats. This will not be an issue for some people, in which case I recommend this hotel unreserevedly, but if you like a little living space - some chairs, wardrobes, and so on then you might want to book elsewhere.
We struggled a bit with the lack of space but we enjoyed everything else about the hotel and would certainly stay again.