Pros:
Extends my range since I can travel further with more water.
Travels on my back, out of the way.
It has a pocket in which I can put snacks like a banana, Clif bars, and small bags of Gatorade powder for my water bottles. The ice water in the bladder keeps snacks cool.
Wide mouth on it making it easy to fill.
Easy to unclip the tube from one of two plastic D-rings and drink while moving.
Mouthpiece requires biting down in order to open, so no leakage if you lose control of the tube.
Pack has some reflective strips on it for added visibility at night.

Cons:
Can be a bit heavy at the beginning of a ride.
Often makes my back sweaty.
The pack isn't heavily insulated so water doesn't stay cold for long, even when filled with ice water. When drinking, have to suck on the tube for a bit for the water to turn cold.
While the tube is easy to unclip, sometimes hard to clip back onto the D-ring.
Is designed to only hold water, nothing else, due to difficulty in cleaning.
After use, have to hang it on 1/2 a clothes hanger for several days for it to dry completely so mold doesn't form in it.

But even with these cons, they're all minor to me and far outweighed by the advantage of having extra water when I need it. I tend to sweat a lot, and this hydration pack has been a literal lifesaver with 2 liters of extra water on long, hot summer rides in rural areas with absolutely no place to get water.