My friend and I did the full circuit and "w" trek around the park and we of course loved it. If you can, I suggest beginning the hike where the bus first drops you from Laguna Amarga, as you you get away from the crowd you get dropped off with and it's a nice hike for the first day.
The back side of the park has fewer people, but the campsites are still quite full at the end of each day. The mosquitoes on the first three days were a nightmare!
Once you get all the way to refugio grey, you start hitting the crowds of "daywalkers" (we use the terms of people out for the day, without full pack and not camping) which can be annoying. The sites were all crowded from this point forward (even with camping facilities), but we managed to figure out the times when people would try cooking, hit the trails in the morning, etc, and managed to avoid the "masses" most of the time.
To be clear, the trail itself was great, the views amazing, and it was a fantastic experience. It would be nice if they limited the amount of people in the park, but given the fancy refugio system, i don't see that happening anytime soon. Appreciating this is from the perspective of a backpacker who wants to camp and not see very many other people. The park is remote, but you're certainly not along :).
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