Hi there, and welcome to the forum.
I have had an 2008 F650 twin for the last two years and now have 42K miles on the clock. I bought the bike s/h with 7K miles on the clock.
As others have said, all the early problems have been sorted out through recalls. I have had the head bearings changed once, but I was giving the bike some serious off road abuse at the time.
The only real problem I had was when the lambda sensor failed at about 38K miles. It took BMW a long time to work out what the problem was and one dealer never did! The symptoms were a rapid increase in fuel consumption, (from 60+ mpg down to 45mpg, and the engine choking up at around 4200 rpm). I only know of one other person who has had this problem, and once diagnosed properly, it was easily fixed.
You will definitely need to change the screen and for any decent miles get a gell pad inserted into the saddle (or use the air hawk).
The biggest problem I have had with my bike is the vast array of goodies that can be bolted on! Its like a money magnet! Bash plates, crash bars, lights, huggers, fender extenders, panniers etc etc. But the basic bike is great. No one has mentioned the fuel economy...which with the price of fuel now is a major bonus for long trips and daily commuting. Though the tank is relatively small, it still gives a range of over 200 miles and the all the weight of the fuel is low down beneath the saddle. Normal fuel consumption is 60 MPG upwards depending on your style of riding.
It also runs on tubeless tyres which make blow outs less dramatic and easier to fix. Having said that, I have never had a flat. Tourances have been the best tyre I have had so far. (Non EXP).
No one has mentioned the G650 single. I have never ridden one but I was pretty impressed with the performance on a ride out of over 30 bikes yesterday from Rainbow. Most of the bikes were 1200GSs but the little single didn't seem to have any problems keeping up.
I'd be interested to hear other people’s experiences. But for the moment I will stick with my twin until it falls apart. It's like my best friend now. I will be doing my first two up touring with it in a few days and I'm sure will appreciate the extra power over the single.
The twin is reputed to be a good bike for beginners, which in my book also means it still feels friendly after long days in the saddle. I’ve only ever managed 1000 miles in a day, and frankly wouldn’t want to repeat it. It was before I had the gel insert and I think I only ate chocolate and coffee that day! Sore arse and weird head!
I’ve not tried any of the other bikes mentioned so can’t comment.
Good luck with your buying choice.