The ride up to the Rothorn in the little red train is absolutely amazing, and the panorama from the top is simply incredible. You can practically see across the entire country!
I'm less convinced about the hotel and restaurant, though. Our room was in a side-building, and smelled really musty. If you want a towel, you have to rent it for 4 CHF. The dinner in the restaurant was ok. At breakfast, however, the fruit salad came out of a can, and the bread had been deep-frozen. There was nowhere in our room to play a game after dinner, so we asked if we could stay on in the restaurant. The answer was no, as they had to clean and then wanted to leave (it was barely 22:30). Having just come from Lake Oeschinensee where no effort is spared to make guests feel at home, the Rothorn Kulm Hotel was a big disappointment. Especially given their prices, which are high.
However, if your prepared to lower your standards, I would still recommend you spend a night there (ask for a room in the main building, they're supposed to be nicer). Being perched this high on top of a mountain, seeing the sun setting and rising over the alps, with absolutely no sounds but the wind and the birds, is a very unique experience.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.