Maybe I'm the worng one to jump in with an answer to your question, but here goes...
Nancy & I went on Celebirty Infinity from Buenos Aires to Santiago last February. Pictures are at http://ricktalcott.smugmug.com/Travel
if you want to save reading a thousand words or so. The best part of our cruise was the Beagle Channel and the Chilean Fjords. It was a fabulous trip, scenery wise.
We picked Celebrity Infinity because my wife is prone to seasickness, and we thought that a large ship of ~2000 pax would mean that the ship had less rocking. In that we were right. If I were doing it all over again, I'd spend some time in Buenos Aires and then fly on to Ushuaia and take a smaller ship for the cruise portion of our journey. The three days at sea getting to Tierra del Fuego and the three days after the fjords were just "days at sea." Nothing special to that.
Nancy has been to Machu Pichu, and she wouldn't recommend taking a shore excursion from the ship to Machu Pichu. You would be going from zero elevation to 12,500 feet, and the chances for altitude sickness are quite high. Ideally, you need a day or two to acclimate to changes in altitude.
I wouldn't go to South America for the glaciers. Glacier Bay in Alaska was far better.
Celebrity really had its act together when it came to entertainment in the evening. Tango and gacho shows etc. Lectures were good, but not great. The food did NOT connect with the destination. All in all, the cruise could have used the services of an editor.
Our best shore excursion was for the penguins. Make sure you see those. Other stops, such as Puerto Madryn in Argentina and Puerto Montt weren't worth throwing money at.
If I go back, I will make sure to spend some days at Torre del Paine in southern Chile. One of the disappointments we had was that we simply didn't have enough time to fit that in on either end of the trip.
Let me know if you have any questions.