I test-drove a 535i GT twice before ordering mine... my first impressions were that the car was rather large (partially due to sitting much higher up than my current 650i) and that it maneuvered more like a 7 than a 5 sedan (probably due to being longer than a 5). Mine arrives in two weeks, so I can offer a more informed opinion then.
I ordered everything on my GT except:
night vision (could not find a single car -- of any model -- in the SF bay area to test drive it; too expensive; did not think I would realistically take my eyes off the road at night to watch out for pedestrians in the iDrive screen)
active cruise control (too expensive given how rarely I'd use it)
executive/luxury rear seating package (flexibility of a fifth seat outweighs nicer rear seats)
ceramic controls (did not seem to fit with the GT's interior aesthetics, seemed overpriced)
smartphone integration (did not want to sacrifice center console space, seemed redundant)
The one must-have seems to be the camera package, given how poor the rear window visibility is. Every GT I've seen on a dealer lot has had this option, no matter how tricked out or stripped down each car is configured.