I have ridden (but not owned) a 2009 KLR. Nice bike butt my impressions are that its quite tapped out at highway speeds. It will do more than 65-70 mph but it sure is not fun. The RPM's are high, the vibration is extreme and the bike feels like its going to grenade under you. I may be exagerating some but you get the point. This bike is a good friend's, and he categorically does not like riding in excess of 65 mph on this bike. It's fine at 55.
I own a 2009 BMW F650GS (the twin, usually known as the F658). While it is in the same class as the KLR, it is a whole different bike. I regularly cruise comfortably at 75-80 mph. The power is such that quick passes are possible, and hitting an even 100mph is not difficult. The LKR has something like mid 30's HP. The F650GS has low 70's. It is a BIG and noticeable difference. The ride is different too. At 70-75 mph I kick mine up into 6th gear, the engine settles at about 4000 rpm, and just seems to lope along effortlessly. The ride position is very comfortable for me, and with the aftermarket corbin seat this is the "perfect" bike for me.Its the first bike I can realistically ride tank to tank. the heat grips rule (much warmer than most aftermarket grips).
If it means anything the KLR owner wants to sell his bike and buy a gs.
To be fair some of the klr vibes may be due to tires. He's running 50/50 Mefos. I'm running 90 street /10 dirt michelin anakees. His are semi knobbies.
if you want a primarily street bike, its very very hard to go wrong with the F650gs. I'm coming off a Kawa C14 supersport touring bike with 155 hp. I am in no way bored with the F650gs. It handles like a sport bike (it's light and nimble). If you do want dirt riding capabilities, look at the f800. A bit more power (not rally needed) but its more dirt oriented wwith spoke wheels and a few other differences.
I'd buy mine F650 again in a NY minute. Maintenance costs ar higher, but check intervals. Oil changes, for example, are every 6000 miles instead of 3000-4000 like many other bikes. I'm not finding mine to be extreme. For what its worth, I ride 4 hours to and 4 hours from the local BMW shop and I dont mind the ride
I have no experience on the single cylinder BMW's. Info from others (second hand) is that they do not compare.