A whimsical modern building in a Baroque area.

Contrary to popular belief, It was actually designed by the Croatian-Czech architect Vlado Milunić in cooperation with Frank Gehry. It was built on a vacant riverfront plot accidentally bombed by Allied forces.
The house was (and is) controversial because it stands among the Baroque, Gothic and Art Nouveau buildings for which Prague is famous. However, the design was avidly supported by the then Czech president, Václav Havel, who lived for decades next to the site. Beauty is in the eye of the beholder, and I think it works well.
There's a special exhibition area on the lower three floor, and a ticket gets you to the observation deck as well; otherwise, you can pay the same price or buy a drink apiece at the top. Judge for yourself from the photos whether it's worth it.

Beautiful building and beautiful view from top
The building is beautiful. Great photo oportunity though it's hard to get a picture of you in front of it because it is set on an intersection. Go in the building and take the elevator to the bar on top. If you want to go out to the terrace to enjoy the view, you have two options. You can pay just to enter or you can pay a drink from the bar and enter for free. We bought two sodas (110CZK, around 4.5 euros), which was cheaper than paying just entrance. The roof is round with mesh sculpture in the middle. We drank our sodas while enjoing the view of the whole Prague. And we made great pictures!