Pros:
- easy to get to. This is critical is you are punctured on rough terrain or mud. How are you supposed to drop an underslung spare if there is no clearance?
- easy to get off. My spare is 110lb. When I'm feeling great it is easy to lift off the holder and drop to the floor but much more tricky to get the flat back up in place - harder still if I'm worn-out. Mine has a drop-down feature; it can be wound down or lifted down; much easier to use when I'm tired/the weather is really hot. I see lots of guys with big spares on the roof; 2 man job to get them up or down safely. Once up they must be very soundly secured for obvious reasons.
- easy to take entirely off to lose weight for driving around town. A whole section of my carrier can be removed just by pulling a pin.
- safe. there are way to many spares thrown in the back with no/inadequate tie-down. In a crash or sudden stop the spare becomes a deadly weapon.
Cons:
- getting into the back of the truck is a little more of a chore. No biggie.
- limits what other stuff you may want to put on the back
- can't see ******* out the back. My wife bought me a reversing camera for Christmas - yet to be installed. For me, I try to keep my right rear side view as unobstructed as possible when packing the truck and I pick parking spots very carefully.
If you buy one get a good one that bolts to the bumper. Make sure you bumper is big enough for the weight. Get one that is VERY securely attached at the swing point, has grease nipples all over it and has a good locking mechanism. Mine also locks in the out position to stop it swinging around in the wind/on a slope. You get what you pay for.
When you connect it to your truck, make sure you torque the bolts properly and recheck the bolts every time you've been off-road. There is a lot of weight cantilevered off the back putting stress on the fasteners.