Been flying Gulfstreams for 10 years now, GII, GIII, G550 and now G650 and I can say their dispatch reliability is excellent, in all that time I've only had 2 failures that grounded the plane and interrupted a trip.
The new G650 is exceeding it's range predictions; Originally predicted to fly 7,000nm at M0.85 and 5,000 at M0.90, now reportedly achieving 7,000nm at M0.87 and 6,000nm at M0.90.
The cabin issues with the GCMS system on the first 50 or so planes has deffinitely been a problem but seems to be settling in now and more and more crews that I talk to say they are having very few if any problems with the cabin. The first 650 our company took delivery of was in the first 40 and did have a lot of issues, but are all worked out now. We are now taking delivery of two more this month which we have been monitoring closely through completion in Savannah and so far everything is indicating that the delivery of these two should be a lot smoother.
I know that completions at the Long Beach facility are not going as well, indeed I have an associate who's aircraft is 10 serial numbers ahead of ours but won't be finished by Long Beach until May.
As with any new aircraft design there is a lot to be said for avoiding the first 50 (or 100) off the production line.
In my personal experience Gulfstreams product support has been excellent albeit not cheap.
I personally don't have any experience with Bombardier products but a good friend who I attended G650 initial training with flew a Global for 10 years and stated that he was not happy with the reliability of the aircraft, but others I have spoke with have nothing but praise for the aircraft. It all comes down to what best suits your mission in the end.
That being said, when it comes to new aircraft designs, it appears to me that Gufstream has a habit of
"Under promising and over delivering" and that Bombardier seems to do the opposite as evidenced by the Global Express entry into service.