Pros:
They tow smooth and stable.
They often offer a higher load rating.
Tandems offer 4 wheel brakes (as an option) for added stopping power and safety.
They look cool! 
Cons:
Tandems are often heavier.
Tandems are MUCH harder to turn sharply, by hand or when coupled to your tow vehicle.
Twice as many tires and possibly brakes to replace when needed.
Added length.
There’s others on both sides of the slate. I’ve owned one tandem that was a horrible product put out by a up-start company. That being said, if I could afford it at the time of purchase, I’d go with the tandem over the single EVERY time given the amount of miles I tow.
I have a question:
If you blow a tire on a tandem, will the other one support the tire until you get somewhere off the highway/convenient to get the first repaired?
Gianni, was wondering that myself.
James (or others), what about launching with a tandem? Do you need more ramp length or more submersion?
Also, was wondering about backing up a tandem into a garage or storage where you have a tight space you are trying to get into? Anything tricky there?
Sorry for the dumb questions but human nature is to hate surprises, especially after spending a bunch of money.
I fished with a guy out of Green Bay who had a tandem axle trailer under his big boat. We got to the access about 80 miles from his house early one morning and as we were taking the cover off his boat I said “You only got one tire on this side” Of course he thought I was joking, because he had just replaced one after losing it on his way home from LOTWs(he thought I heard about his prior trips occurance and was giving him a hard time). I mean the tire, drum and all. The only thing left were the brake shoes.He said he noticed a little change in his gauges on his deisel but didn’t think anything much of it. We trailered it home and stopped and picked up a stub, cut the old off, welded up the new one,put in new bearings and drum, threw on the spare and off we went again. So evidently you can still pull with one tire on some trailers, don’t know about all, but think of the risk your taking. All the way back that evening I kept watch along the road for a tire stuck somewhere it didn’t belong, like through someones picture window! 
I have had both and would choose the tandem over a single axle.
The trailer follows the tow vehicle much better, no swaying.
I have surge disk brakes on only one axle but if buying a new one would prefer brakes on both axles. My current setup will stop my truck(which is 6500 pounds) and boat trailer (an estimated 3500 pounds)in a shorter distance than the truck will stop by itself.
Never had a flat on any boat trailer but if I did and did not have a spare, I would not hesitate to drive a few miles to get the tire repaired without removing it. Lots of factors to consider though! Reduced speed, other conditions.
Backing up a tandem requires a slightly different approach as the trailer does not react as rapidly to changes in tow vewhice position as a singe axle trailer does. As far as i am concerned that is a major positive characteristic, but for some situations it could be negative.
Trying to move the trailer by hand: is easier when just rolling it forward or backward, but if you are trying to position it to either side, forget it, it is next to impossible to pivot a tandem by hand. (Would surely depend on the weight of the boat.)
I have never noticed that the ramp length has made any difference.
Depth into the water is a function of ramp steepness and how low the trailer is. Most tandems I have seen are dropped axle types that have the boat lower to the road, which equates to floating off in shallower water.
Lastly I agree with James, two axles means increase carrying capacity, easier towing when properly balanced with appropriate toungue weight, should improve towing MPG.
My rig is a 20 foot Ranger 488 w/200 HP and a tandem axle Ranger Trail. Pulls like a dream.
Hope this helps. 
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