We adopted a Great Pyrenees from a no-kill shelter when he was over a year old. We were the third family to adopt him. He went from being an unloved, unwanted city dog to the guardian of our family members, home, and eventually 2 acres, a cat, another dog, and poultry flock.
Adults dogs have a history that will require patience and love and work to understand and correct, if needed.
They do bark......after a while you will be able to tell if it is the "Ok, world, I'm on duty" bark (throughout the day), or the "hawks are scoping out the flock" bark (as needed). They both sound differently from the barks and growls that alert you to people or animals near your fenceline or vehicles in the farm drive. The advantage was that most people did not venture beyond the drive gate....because who wants to test the barking, growling, 125 pound (looking 150 pound with all that fur) dog?
They can get along with other family dogs and cats....but a strange dog on the property will (ime) be treated as a potential threat. When our friends came to visit, they did not bring their dogs. If they were driving by and pulled in to say hello because we were out in the yard, and happened to have their dogs with them, their dogs stayed in the vehicle. It was Titan's property to guard, we did not allow other dogs to mark the property. We adopted a rescue Brittany Spaniel a year after we were on the farm. Titan added Maddie to his list of guard duties. He was the one who really alerted us that something was wrong with her right before she was diagnosed with cancer.
They do need fencing. If they don't see a border, a Great Pyr will consider anything within view as "property to be guarded". If your property is fenced, don't leave the drive gate open.
As working dogs, they do best when they have a job to do and our experience has been that if you don't give them a job, they will create one or get into mischief.

They are very intelligent and learn quickly. Whenever we moved to a new home, we walked the perimeter with Titan and talked to him about what his job would be. Sometimes that was more difficult than that sentence relays. When we got chickens, we let Titan sniff them and told him it was his job to guard them. We had no problems.
Our ds and dil also adopted a Great Pyr. They are currently living in a large apartment, but with walks 3 or 4 times a day and play time at the park, it seems to be working for them.
There are a lot of great resources for people considering owning a Great Pyr. Here are a couple I like...
http://www.greatpyr.com
http://www.great-pyrenees.us
Titan recently died at 12 years 8 months of age. While we had owned dogs before this, Titan was our first experience with the Great Pyrenees breed. We have decided we will probably get another Great Pyr when we are ready.
If you are interested, here are two links to my journal posts that describe in detail our experience adopting and living with an adult Great Pyr. Pictures are included.
http://www.sufficientself.com/forum/viewtopic.php?pid=18290#p18290
http://www.sufficientself.com/forum/viewtopic.php?pid=244616#p244616
Good luck with your decision...........