I like them mostly because the have an endearing personality. They have a soul that the I4s don't, IMHO. I also like the classic look of the cylinders sticking out each side. It's a very wide motor, looks cool. Twins have good low-end torque and drive out of corners nicely. The powerband is nice and flat, so it's user-friendly and you won't have a sudden rush of power that can cause trouble if you're healed over. Also, their powerband is from 3-8,000 rpm; no winding out to get to the good stuff.
Cons include significantly less maximum power (although torque is on a par), and vibes are much stronger. But twin vibes aren't nearly as unpleasant as I4 vibes, IMHO.