Hey Tim,
I've never owned a K12RS but spent a little time on them and was interested in them when they first came out. Being a twin guy, they just weren't enough to get me to buy one. Their weight, as I recall, is about 630 lbs. They are pretty fast and were comfortable imho, with large bags.
The K12S weighs 575lbs on my scales ready to go. It is long and hard to turn quickly. Unless you ride really fast on the street you may not find this, but I had mine on the track and it got pretty squirrelly when pushed in the transitions. Being designed as an autobahn burner, it can't run with an R12S when the going gets twisty....as least not with me on it. Mine has the electronic suspension which I like for street duty, but it really needs more if you track it. It might be more of a twisty bike if you didn't get the ESC but the stock suspension and upgraded to Ohlins. But....on my trip out west it was nice to be able to go soft while my mates were having their kidneys pounded on their Ohlins sprung R11S/R12S bikes.
The motor is a monster over about 6k with a redline of 11k. You can spend all your life with it below 6k and be happy as a clam but once you get it revved it really accelerates. But....it takes a moment or two to get there, so for passing fun, the R12S is more fun as it does it "right now". Spend time between 7 and 11k in any gear and you will have fun, but not in jail.
I use my K-bike for touring duty. Although the bags are smaller than those of the R12S or the K12RS, the rear seat gives a good platform for lots of Helen Two Wheels bags. I spent 25 days on it last summer and found it smooth and pretty comfortable, although my 63 year old knees were complaining at the end. Not sure if that was all the camping or the ride, but I was glad to be off it when I got home.
Maintenance is pretty simple although I let the dealer do the valve service at 23xxx miles. Supposed to be at 18k but I was on a trip. Two valves needed adjustment and then not much. This is the first and only bike I've ever bought extended warranty cvoverage on, so I hope to put 100,000 miles on it. It will get the nod again if I go on another trip.
The tranny is chunky, but the later ones....2007 and on are better I've heard. My 2006 requires an educated toe to shift smoothly. Not something you can't learn quickly. It will never shift as smoothly as a Ducati. That said, the smoothest BMW tranny I've ever owned is my 2007 R12R. I wish they all were like it.
That's about all I can think of. To lurk a good site and learn stuff go to
I-BMW.com; The K1200S, K1300S, K1300R, K1300GT, K1200GT BMW motorcycle Portal, Powered by vBulletin
PS. What happened to Tims' question about the K-bike? Deleted while I answered? Oh well. I needed to practice my typing anyway.