I have a 1150R Rockster.
They are a great standard bike, they do nothing great but do everything well.
Like most BMWs if you are not 6' or taller you won't have both heels on the ground at a stop. Properly maintained they can go for hundreds of thousands of miles. At the BMW ralley I went to last year there were several with 300k+ on the original engine.
Under 20mph is shows its weight, over 70mph I wish it had a better wind screen.
After-market stuff is expensive and so are replacement parts, luckily they are engineered well so you probably won't have any major repairs until you reach 100,000 miles.
The dual spark engines run better, changed sometime in 03-04, the single sparks tend to load up at lower RPMs. If it has ABS it takes some getting used to, it's servo assisted and imho overly complex, mine does NOT have it and I'm happy about that. Heated grips FTW, most beamers have them.
Things I've learned:
The engine has a alot of torque so you'll be tempted to lug it around at 2300rpm. Don't, 6th gear shouldn't be used under 70mph and try to keep it above 3000rpm, cruising around at 4-5k is perfectly fine and you can do it all day, the engineer who designed this engine said so.
Tires make all the difference, The bike was night and day better once I put Pirelli Diablo Stradas on. Buy these for you next set, thank me later.
Any Questions just ask.
-JTP