This is the perfect place to stay in Prague.
Martin & Petr are the perfect hosts to have.
You could look at other apartments, but I don't know why you would, because this is the best you can get.

Martin responds to every email promptly, and once you arrive and meet him or Petr, you will find them endlessly gracious and helpful.

The apartment is beautiful, spacious and scrupulously clean. You will feel like you are staying at a friend's place who happens to be out of town, given that there are nice personal touches like books on the shelves, coffee in the kitchen, etc.

As others have mentioned, this apartment is as central as you can get: you will look out your window at the famous astronomical clock. From this location, you can easily walk anywhere, or catch the Metro.

If you are especially sensitive to noise, be advised that this apartment overlooks a central and busy spot. But, when it got to be after 10 PM or so, and I closed the double windows, I didn't find it disturbing in the least.