In general, I have found gaited horses to be excellent trail mounts because of their utterly smooth ride and awesome ground cover.
Personally I don't think you can get much calmer than your standard Quarter Horse. It can be strange going from non-gaited to gaited. I have found that gaited's have a quicker stride and it may seem as if they are constantly 'rushing' when they are really just moving along at a regular gait. They DO take some getting used to.
Gaits pretty much eliminate saddle soreness, even on long rides
Can cover ground quickly and efficiently
Tend to be flashy with a pretty head and tail carriage
Some gaited horses are never taught to canter, and hence only walk/trot.
May irritate trail partners whom's horses don't stride like a gaited, and have troubles keeping up
It can take time to be able to identify a gait and keep a horse in it. Gaited's can trot and some do. Many gaited horses have multiple gaits, which can be difficult to identify.
Gaits are easy to identify, such as walk-trot-lope-gallop
In general, slower and easy going, make for a relaxing ride
Trail partners don't usually have troubles keeping up.
Very high chance of being saddle sore after a long ride
May not be able to keep up with fast-walking gaited's
It may seem like the gaited horse has a lot of 'cons'. In reality, their just different. I would recommended a gaited horse to anyone, especially older people or people with past injuries that can ache because of a bouncier ride. I love riding my gaited's, it's smooth, it's comfortable, and I adore how they display themselves.
I broke my back about three years back. I rode a QH with a very smooth trot on a trail ride and I was sore all over afterwards. I ride my Foxtrotter all over the place, and hop off with no aches or pains.
Unless he does something bouncy...like spooking or trotting