|The G650 is easy to spot. When looking at it, one can notice that the cabin windows are about 30% larger than on the G450/550. Also, it's a much fatter aircraft, finally matching cabin volume of the Global Express.
The easiest thing to spot though is the cockpit wrap-around windscreen, which has a much rounder and better integrated design, compared to the short, somewhat angular design on the G450/550.
On the issue of taxiing with one of the buckets open: that's normal procedure for the Gulfstreams. both engines are at idle thrust and it beats riding the brakes. Carbon brakes suffer badly when riding them.
The 650 is truly an amazing aircraft. On a recent trip, we flew our G550 from Hong Kong direct to Vegas, taking of around 40 minutes ahead of a G650, which was going the same way. They landed almost 40 minutes ahead of us, cutting a flight time from our's of 12.5 to their's of 11.1!
Basically, we were cruising at M.80 and my best guess would be that they were doing about M.89.
On a whole, the most amazing thing about any of the big Gulfstreams is, that they carry all their fuel in the wings as opposed to others (Challenger/Global), which have a multiple tank system. On top of that, the Gulfstreams have a very clean wing.
They are fun machines indeed, that being said, I luckily don't have to do ultra long haul at ungodly hours very often.