I've never flown the Legacy 600, but I have been in the preliminary stages of research for a company who wants to buy one. The previous posts are mostly correct, it is essentially a 135. However, it has, in addition to the ventral tank a third or fourth fuel tank aft of the baggage compartment. Basically doubling the fuel capacity over that of a standard 135. It also weighs about 5,000lbs more empty, mainly because of the interior comforts and ammenities. It also has the A1E engines rather than the A13's that the airilne versions use. I'm sure there is some more, but those are the main differences that I can gather. Good luck!!

Biff