Stayed in the hotel for close to a week and I must say that the service is great!
The staff are attentive and very friendly, always willing to help.
For the price, Zen Hotel offers a very good deal with comfortable bed, nice room (I stayed in a suite room, with toiletries all provided for), and good breakfast. House-keeping is done daily.
The hotel location is also great considering that it is only a very short walk to Hoan Kiem Lake (the lake is not very big, and many attractions are located near the lake). I saw the location of the hotel somewhere in one of the internet booking sites and was pleasantly surprised when I reached there - the hotel location was actually much more accessible than shown on the map. The hotel provided me with a good starting point to visit the rest of the city and the Old Quarters, which is also a short walk away (the busy Hang Gai, Hang... shopping areas are close by). Even Church Street is very much within walking distance ( > 15-30 mins), and the city is best explored on foot.
Advise: Get a room away from the streets if possible. Streets in Hanoi are noisy as early as 5am with all the honks blaring in the streets. Other than the locals, most tourists will find the noise quite unbearable and advisable to get an inside room even if they say that the rooms are quiet (by their standards).
Highly recommended!