I stayed here for four nights and it was great. Communication throughout was excellent and if you're flying into Prague airport they give clear instructions that will get you there inside about 45 minutes. If you're arriving by train into the main Prague station, it's a simple 10 minute walk.
Other reviewers will have mentioned the lack of a window and the fact that you have to negotiate quite a few sets of stairs (it's on the top floor without a lift) but that's all stated very clearly in the description and shouldn't be grounds for complaint retrospectively.
Overall I recommend staying here if you're a solo traveler who needs an affordable and excellent value for money stay in Prague that's located in a great area close to some amazing bars and only about a 20 minute walk from some of the central main attractions.
Prague's a beautiful and enchanting city and I look forward to visiting again some day. When I do, I'll happily stay at Mark and Marcela's place again.