I have logged over 1900 hours on Express (20), 5000 (150), XRS (1650) and 6000 (200) and never got AOG.
Most fragile components are more related to completion than airframe design, the water system was an issue on one particular XRS but avionics were never an issue.
It's a highly computerized jet and some resets are sometime necessary, although very seldom requiring a complete shut down.
Preparation time: I personnally like to reach the aircraft 2 hours before departure if not duty time limited with 1H30 as a minimum when with pax.
I guess I was lucky to have flown well operated and maintained aircraft but I do believe that the Global fleet reliability can be considered as similar to the V/550's nowadays.
Also I would always recommend hangarage, especially when the temperature drops beyond -10°C.
Bombardier support is generally very good.
Having wrote that the Gulfstreams are also great from what I heard. An important thing is a well conducted pre-buy inspection in any case.