The scenery is stupendous - jagged mountains, turquoise lakes and huge glaciers. Very unspoilt and an ideal place for hiking. Highly recommended, in spite of the very strong winds!
However, be warned if you intend to visit it without your own transport, as we did, and are not intending to do the long hikes (which take several days) and camp. Getting around is difficult.
The park is 3-4 hours away by bus from the nearest town, Puerto Natales. All accommodation within the park is very expensive. We stayed at the cheapest hotel in the park, the Posada Rio Serrano, at the southern end, very close to what is called the Administration Centre. The hotel cost 125 US dollars a night and payment had to be made via fax. The room was no better than a hostel room, of a very poor standard and certainly not worth the high price. Breakfasts were very frugal and everything rationed, even the hot water for tea and coffee!!
The park is huge and public transport within the park almost non existant. Several bus companies run buses from the Adminstration in the south to Laguna Amarga in the north (and then on to Puerto Natales) but all leave at the same time, 1pm!! Similarly coming back, all buses leave Laguna Amarga in convoy around 4.30pm. Why don't they stagger the departure times, instead of all running together? There were no buses from the Adminstation to the Lago Grey (18 km). We ended up hitching a lift.
Make sure you have done your homework before you get to the park. Decide where you want to go and what you want to see. There is little printed information once you're in the park, apart from the map you get when you've paid your 15,000 pesos park fee. Our hotel had no leaflets about the park and no information about excursions organised by other hotels e.g. boat trips on Lago Grey to the glacier run by Hotel Lago Grey. (The man at reception couldn't even find the telephone number of the neighbouring hotels.)
One other piece of advice: bring some food with you because you can buy very little once you're in the park, apart from a few tinned items and snacks. There is a good supermarket in Puerto Natales and two shops that sell fruit.

like no other place I have been. you have to go at least once in your lifetime!