Pros:
opened up the market for people that wouldn't/couldn't ride otherwise (smaller individuals [Hey, that is me!], women that wouldn't ride a two-wheeled bike or couldn't due to size, those with health issues)
unique and nice looking Well, unique for sure.
you'll meet your neighbors and many strangers (conversation starter)
fun as heck!!!!!!!!!!!!!!
less dangerous than a two wheeled bike (easier to balance too)
tons of parts to individualize it (though sometimes it seems it would just be more efficient to duct-tape Gold Bullion bars on it)
storage space is handy
if you've never rode anything, you can do this from day 1 (except for that one rider who totaled a Spyder in the Sioux City dealership parking lot while doing a BRP Demo Days course)
options of semi-automatic or manual
reverse
loud horn
probably more visible than other bikes
cons:
expensive to buy but very cost effective when it comes to getting smiles
gas mileage, my car gets better gas mileage Yeah, should be better.
steering issues that are unresolved
so much computerized crap that it's hard to figure the thing out
not set up for a comfortable ride, but you can buy parts to make it so
a good dealer is hard to find but a treasure when you do
expensive service this is my biggest con - cost of ownership
some cheap parts on it that notoriously break