K1200S: the bike bestowed with an AWESOME motor but a crap chassis and brakes. And this from a rider who does not care for 4 cylinder bikes. It doesn't have the sound or engine braking of the big bore Duc twin but much better power down low than most 4's, no FI snatchiness and a very quick revving response with decent tranny...actually a superb tranny by typical BMW standards, just not as fast and short as a Duc or Honda tranny. Unfortunately this powerplant is mounted or encumbered as it were, with typical modern BMW engineering stupidity. Telelever, ESA, ABS, linked brakes, paralever shaft drive....The end result is a chassis that is slow to respond, offers zero feedback and brakes that are wooden in feel and offer zero ability to modulate. I prefer to enjoy the riding experience myself and make my own decisions on how to brake and manuever based on my own skills, not have some German engineer a way the bike he thinks it should respond and perform.
Alternatives to your ST4s? ST3 is certainly lower maintainence and easier maintainence. Not good enough? You don't mention your intended usage other than commuting. If that is your sole usage, I'd look at a MTS 620 dark or MTS 1000s. Both offer a comfortable position for low speed city riding while still offering some room and protection for higher speeds. The 620D is just a phenominally light and flickable ride with the preferred single front disc brake for lighter, more responsive handling and feel yet still plenty of brake for that bike and its purpose. The MTS 1000s ups the ante in power but most importantly you get a phenominal fork that really performs offering spectacular feedback and feel that you'll appreciate navigating city roads as well as blasting through the twisties during weekend play. The air-cooled motors combined with far less bodywork makes servicing an easy and cost efficient task.
Other possibilities:
* SS1000S/SS800s: same easy service 2V air-cooled motors in a more sportbike riding position.
* Monster S2R1000/800 or M620/695: all terrific lightweight naked Ducs with easy service 2V air-cooled motors
* Triumph Sprint ST: great motor, available bags, good suspension though lagging in feelcompared to your Duc, not to mention it is noticably heavier bike.
* Triumph Bonneville: They come is several flavors, even saw the new GREAT looking Scrambler today. All make wonderful, cheap, efficient commuter bikes
* KTM 950SM: Maybe not the most practical and certainly not the most easily maintained but that beast does look ALOT of fun. Hey, you gotta have fun, no?
* Triumph Speed Triple: Only barely lagging behind the KTM in grin factor, but much better maintainence and similar practicality/versatility.
* Honda 919, Yammy FZ1/FZ6, Zuki SV1000/600: ah...well...BORING forget it, pick any of the previous...you only live once and life's too damn short for a boring bike.