Welcome to the forum, alexanders.

In my experience, the GL1800 platform in general is robust and reliable. Oil/filter and rear diff oil changes are super easy, as are plug changes. ReserveBum is correct that the air filter replacement takes a bit of time. It's a "feature".

With regard to saddles, I'm 6 years in and still on the stock one, but I have added a beadrider beaded seat cover (hey 600,000 cabbies in NYC can't ALL be wrong). I have ridden the bike with a Mustang seat, and it's nice, but made for a bit too much knee bend for me (the lower seat height makes the seat-to-peg distance a bit shorter).