Best bikes in the world. (I'm biased).
They are great, but they can be complicated unforgiving pains in the arse. They like to be ridden lots and maintained properly.
Most of the ones you'll find have been taken care of - just be sure to know the history of the bike if you can. If you're not sure of the history when you get one, there are some basic things you'll want to do right away.
Spline lube is up there, which involves removing the rear wheel, swingarm bolt, sliding the swingarm back a bit to then allow you to unbolt the transmission from the engine and slide it back a bit too. This allows you to get in there to apply some grease to the splined shaft connecting the tranny to the crank.
The wheel bearings are ridiculous. They're tapered bearings with about a dozen various spacers/shims etc. in each hub. It's nuts. Built to last literally a billion miles (someone did the math). But as they are tapered, if for example your rear axle pinch bolt loosens off (or falls out entirely), the rear axle can eventually work its way a bit loose, eliminating the crucial pre-load on the tapered bearings, leading to catastrophic failure which can also completely destroy the rear hub which has gears pressed into it to mate with the shaft drive gears. Yup. Don't ask how I know.
To pull the bearing stack out of the hub involves putting your axle in with spacers to maintain pre-load on the stack, heating the hub to 250 degrees and then pulling the stack out for maintenance.
When it came time to replace mine, I sent my hubs to Woody's Wheel Works in Colorado. For $100 they do a complete swap out of the stupid BMW system to a much more sane setup of spacers and sealed bearings. Parts and labor included. For $30 more per hub they bead-blast and clear coat. Honestly, my hubs look like new now. Just about to lace them back up with stainless spokes (doing 4 wheels - mine and a friend's).
Anyhow - love the bike - can ride it for 12 hours a day no problem. It's my long distance touring bike and unless I'm hard up for cash I will never sell it. I put a Boyer electronic ignition, upgraded diode board / regulator (Thunderbird) and swapped the Bing CV carbs for Mikuni VM32's. Polly Heater heated grip bar inserts and she's good for anything I can throw at her.
Got home from our trip to Deals Gap in Oct 2009 and over that winter I got my XS650 finished up, which became my regular ride. But we're getting both the old BMW's back on the road with new wheel bearings etc. so they'll be good to go.
