Owned a K1600GT for 4 years. Amazing motor and a very agile bike for it's size, but still a very large motorcycle more suited to eating up miles in cross country travel.
In 35K miles I had zero warranty issues - none.
I performed all the maintenance, including valve checks, other routine work and tire changes. The total cost for 35K miles was $3,400. It likes tires - eats them for breakfast.
The bike is the best pure touring motorcycle out there but the cost to maintain the bike is expensive.
K12GT slant motor was a really fun bike to own. Much more sporty than the K16GT, but was the least reliable BMW I've owned with over $5,000 of warranty work performed during my 5 years of ownership.