Stevie,
I've just traded my 2000 R1100S sport in for a 2003 R1150GS. (twin spark). Sadly, I had to sell my R100GS as well.
I miss the S, it is a fantastic handling bike when you get used to it. Loads of torque in the right places. It took me and the missus over the Alps, Pyrenees, Picos de Europa etc. without missing a beat on a couple of occasions. Fastest thing around the Grossglockner was an 'elderly' Austrian on a red S, (despite black being the fastest colour).
On the down side were the plastics. Particularly around the fairing etc. They are very vulnerable to damage in even minor spills.
The battery location and access is another problem.All the panels need to come for access. Best to replace it with a Hawker early on. If you do have to jump start it, remove the starter cover for access to the live feed.
You will feel vibration through the handle bars more than on the GS but this varies with the state of the engine tune (valve clearance, throttle balance etc).
On the whole I'm pleased with the 1150GS. The 100GS felt more chuckable because of it's lighter weight, and the 1100S more powerful and faster, but the 1150GS makes me grin, and isn't that what it's all about?
Paul S