I had wanted to visit Chile and the Chilean Fjords since visiting Puerto Montt in 1973. The Fjords can only be visited by ship and it is the easiest way to visit Puerto Natales and the outstanding Torres del Paine national park.
Cruising with Ponant is very flexible. If you want to explore ports of call on your own, you don't have to pay for the shore excursions. Since we had visited Osorno before this was important for us. We took a public bus to Ancud instead of going on the organised tours. However the zodiac excursions in the fjords were included in the cruise price and were very well organised. The small number of passengers meant that getting passengers on and off the ship for excursions was quick and easy.
One good reason for choosing Ponant is the quality of the food on board. The French take food very seriously and we were not disappointed.
Valparaiso has an interesting history and many grand buildings dating from when it was the first port of call for ships after rounding Cape Horn. However it fell on bad times after the opening of the Panama Canal. Cerro Concepcion with its street art and good restaurants is worth a visit - take the quaint funicular railways.
Puerto Montt itself is just a transport hub, but it is the gateway to the Chilean Lake District. Be sure to visit the Volcan Osorno, Lago Todos los Santos and the falls at Petrohué.