My husband and I spent 3 nights at Mondrian B&B: the location is perfect, near both to the train station and the centre af Amsterdam.
The flat is very nice furnished, you can find everything you need, and even more, and is absolutely clean. Breakfast was great, everything of good quality and not the cheapest you can find somewhere.
Margot - and her son, who welcomed us - are very charming and helpful, we had a wonderful stay, just...too short!