Well , I'd say Harley was instrumental in making BELT DRIVE a standard thing ...
So
Cheers to Harley !!!

Years ago I had a 1980 Belt drive on a Kawasaki 440 LTD .
I loved the clean-ness of the Idea
Till I attempted a "Wheelie" one day , The cords didn't break but ... the teeth all slid into a solid lump of "No Teeth" with 5 bare cords showing themselves where the teeth used to be.   Good thing I was close to home !
After the second belt doing the same thing ... I converted to chain drive and lived "Happily ever after" till the motor started using oil too much ... then the young neighbor kid lived "happily ever after" till he wrecked his $300. bike in a curve.  
We all go through the learning curve , some fast than others  ???

The next belt I had was the first Savage , we all know how "bulletproof" they are.   Then a couple Harley's with Belts they are bulletproof too !

After having two Moto Guzzi bikes with shaft ... I thought the Shaft Drive was a step up till I started getting "Drive Train Lash" after 50,000 miles or so ... Then it was kind of "Dang , don't anything last forever" .  
Yes , I was reading on the Guzzi Site about other folks having SHAFT Problems ... I got rid of mine before any major event but I could tell the potential was growing with every passing year.

The good old chain ... keep it oiled and adjusted and all is well ...   ... till its not oiled and adjusted , then repeat , then repeat , then repeat ...

So look what UPS brought for me today ... Its a Belt Drive for the 650 Meteor ... I'm itching to put it on , but I'm going to wait for the rear tire wear bars to come the the top so I can kill two birds with the same shop time.     The swing arm has to come off for the Belt to go on ...

Am I the only one who has yo-yoed through  the "Final Drive Systems "  ?  

I just want to know how they got that belt to roll up so nice and fit in the box.  It’ll never do that again.  It’s kinda like trying to get your 50’ air compressor hose to roll up neatly.  It can be done, and you eventually learn the trick, but what a pain.

The good ole chain is the obvious mainstay.  It’s proven, it’s classic, it can take some abuse when putting the hammer down.  Power delivery is crisp.

The belt is really nice and clean and quiet and low maintenance.  Great for regular street riding and if you’re not trying to launch wheelies and do a bunch of drag racing.

Shafts are good for car-sized bikes.  Where a belt isn’t strong enough and a chain becomes difficult to maintain.  And an extra 40lbs of shaft doesn’t make much difference in the grand scheme of things, and you’re not gunna be doing wheelies or drag racing.  Great for the Goldwing and towing your bass boat to the lake.

I've had it 'un-coiled" to look at it.   The trick is ... It doesn't coil up in two rings ... It coils up in Three !  

I was looking at my rear tire and the chain wear .  After 6000 + miles the tire is about 1/2 gone and the Chain is 1/2 gone too !  
Going by the wear bars and by the chain/sprocket pull in the middle of the rear-sprocket test.

To put the belt on requires removing the swing arm , so waiting till the tire is gone seems like a reasonable idea ...  

Chains are the most common final drive.  They can handle a lot of power, are "traditional", allow for easy gearing changes, and they require maintenance.  Modern O-ring chains should last 20,000 miles....the sprockets may need replacement at the same time.  Reversing sprockets to move the wear to the other side of the tooth can sometimes extend the life of a chain and the sprockets - putting a new chain on worn sprockets is not a good idea as the distance between teeth has likely changed as a result of the tooth wear and a new chain will not fit the worn sprockets well.

Belts are low maintenance and don't often require tension adjustment.  They don't like rocks/grit/mud and are not used on dirt bikes.  Life span can be 50,000 miles if you ride in a reasonable manner - Hooligans may get lower mileage.  A friend hopped up his Harley and snapped a belt while drag racing and not letting off the throttle between gear changes.  Burn outs and wheelies are also not recommended.

Shaft drive is clean, quiet, a bit heavy and somewhat exclusive.  Shaft drive is not "maintenance free"!  Some bikes require periodic lube of the shaft splines with special grease, and they also require periodic gear oil changes.  A Google search shows that the preferred oil change interval is 12 months/12,000 miles.