I'll tell you the same thing I recently told another new member who was considering the same thing
"you are going to get a lot of info both ways, I am very happy with my 16, there are a lot of members that have expressed extreme displeasure, My advice will come with a few questions; are you a "gas and go" type person, if yes I don't think the truck would be a good fit , are you comfortable doing some of you own maintenance items and learning new things, hopefully yes, do you have any exposure to other diesels????, this part would help too
my truck has been tuned and provides exceptional mileage for a 1/2 pickup, I think they do good with a stock tune or GDE emission compliant tune also if you live in an area that is very restrictive on emissions equipment, some complain about working on them but I have not found anything I would consider extremely difficult, some of the components involved in the tune I chose were somewhat of a "tight fit" and you need some patience.....
the biggest thing I would stress is to not get too deep into this, these trucks do not have a stellar reputation IMO, and it seems you either get a hero or a zero, DO NOT PAY TOO MUCH and be willing to accept the possibility of expensive repairs at any time, read around you'll find members that have enjoyed many miles of trouble-free operation, but sadly you'll find quite a few expensive failures with relatively low milage
best of luck, check truck over thoroughly paying special attention to cooling system and check condition/level of oil, if either seems unusual keep looking, "