Hi,
Me and my friend we rented j&j flat for 4 days and 3 nights in April. It was wonderful! The flat is really large and close to everything ( typical restaurant across the road , bar " ananas bananas " 5 min walking, stores 10min walking ... ) . It have been a wonderful experience! J&j flat is the place to rent when you go in Prague!
Ps: for more fun go next to Karl's bridge you can be inside a bubble on the water , or taking a drink in a bar entirely made out of ice ! I also recommend a little pizzeria where your can buy pizza by slices, on the way to the metro station!
Hope you will have a great time