I've stayed at in a Kras apartment the last three times in Amsterdam over the last ~15 months. If you're going to stay for a week or so (usually the minimum booking period), they're the best option in my opinion.
The Kras has whole blocks of apartments at the rear of the hotel, including on Oudezijds Voorburwal (OZV). Register at the main hotel reception on Dam Square where you will receive a traditional front door key and a credit-card type electronic room key.
Access to the apartments is through to the rear of the hotel. This is not through the kitchens, as you may think, but rather a well-lit and CCTV'd lane leading onto OZV. The Kras appears to own most of this block to the immediate left and many more on OZV itself. The lower the apartment number (I think they are all of the format 73xx), the closer to the hotel itself.
I've stayed in both a regular apartment and a junior apartment, having booked through
www.bookings.nl and have had no problems with reservations. The only hitch I had was in late March of this year when the hotel would only accept a credit card on registration and not a debit card.
The junior apartments are better equipped and more spacious than any other hotel I've stayed in Amsterdam city center and the regular apartments bigger still.
Typically, you'll get a 'fridge, microwave, washing machine, dryer, hot pate, sink etc in a kitchen/lounge, and a sparate bed room and shower room. If you've booked for a week, you'll only get one visit from housekeeping. Smoking doesn't seem to be a problem.
In terms of price, the junior apartments usually go for about E99 a night - more at peak holidays and the regular apartments E120 or more.
Now if only you could book for less than a week..............................