We've stayed for one week in the Sonnenalp Aparttments Building, it was a 2 room apartment it was very nice you have a small kitchen equipped with almost everything you need to cook a light meal and you have a fridge in the room.The only down side is that there is no one at the reception in the afternoon so if you need something you should look for the administrator or receptionist in the morning around 9-10 o'clock.
The bathroom was very nice and clean and running hot water 24/24 . One problem with our room wast that it had a smell in it like an onion or something but i think is because of what the previous persons have cooked because our friends rooms ( 2 ) did not have this issue, but the room was clean.
It is very close (3 min. walking) to the skibus we did not skied in Niderau because there are only a few ski tracks.
I think is good value for money.
P.S. try to go right after the New Year because it is very cheap for us it was 350 euro/week/apartment that is for 4 persons.