Apartment 17 was an amazing place to spend our time in London. We had the upstairs apartment for two visits in December, one at the beginning of the month, and the other over Christmas. The apartment owner was incredibly helpful, and fantastic to deal with. The apartment itself is in a great location - just a couple of minutes walk (if that) from the Marylebone tube and train station, and from there it's very quick and easy to get pretty much anywhere in London. The street is very peaceful, and the bedroom is set at the back of the property - so there was no street noise at all. There is a Tesco Express just around the corner, and there are heaps of shops and restuarants just a short walk away.
The rooms were spacious, and having a full kitchen was great - especially for the Christmas stay when we cooked a full Christmas dinner for us and a couple of relatives. There was all the implements and bowls, pans and equipment we needed (including a full size fridge/freezer, a microwave, an big oven with 4 ring electric hob on top, and lots of bench space as well). Free wi-fi, and free phone calls to a large number of countries too. The apartment was clean and tidy, and really felt like a home away from home. This is a self-catering apartment, so you're expected to supply everything you need, and to leave the apartments tidy and rubbish free when you leave. We were absolutely fine with this, as it is common for these types of apartments - but it is something to consider if you have always stayed at hotels.
Overall we had a great time in London, and this aprtment was ideal for our needs. If you're looking for something with a bit more space than a London hotel room, and for the same (or cheaper) price, then this is for you!