I've owned an F650GS and found service and maintenance work to be considerably more difficult than on a typical single cylinder bike. Oil, filter, and spark plug changes aren't much more difficult, the main difference from doing say a KLR 650 and DR 650 (both of which I've also owned) being the need to remove the false tank panels and drain the remote oil revervoir, located under the panels. Valve adjustments are considerably more difficult, especially on an ABS equipped bike - the brake fluid lines running from the rear of the bike get in the way on the ABS model.
The F650GS is a somewhat complicated bike with fuel injection, meaning lots of associated electronics, sensors, a fuel pump, etc., and there's a lot of stuff packed into a small space, sometimes making for access problems. Please don't take this a a criticism of the bike - it's an excellent bike and I enjoyed mine very much, putting 24K happy miles on it. But, it isn't the easiest bike in the world for a home mechanic to work on, and dealer service is also more expensive than most.
I second the recommendation above to go on the F650 owners website, (they call themselves the "Chain Gang"). There's a huge amount of info available there.
Hope this is of some help.