Owning the three, I think I can share my point of view.
Each car has its own role, so the three are delicious, each in a different way, what its surprising as the 550 and 575 seem so similar at first glance.
The 550 is very well planted on the road, the manual gearbox enhances the driving experience compared to the latter siblings a lot, its a little bit richer in low down torque compared to the 575 but the 575 is sharper at the higher end of the rev-counter. Chassis-wise, 550 and 575 are confortable and supple at cruising speed, but on the limit the differences appear, maybe more evident in my units as the 550 has 18" rims and the 575M has got the modular 19"s. The 550 filters more, the fatter side walls deal better with bumps and ruffled tarmac, while the 575M on 19s makes them more present, and the 550 tends more to understeer, so its safer maybe for the average driver. The 575M reveals itself as quicker, more neutral, more balanced proposition. Yes, its more direct and input-wise, so feels a bit more nervous but get things right and its faster around the bends, its more focused, yet it needs a little bit more care and attention when driven fast. The F1, by today´s standards is dated. The worse double-clutch or six/eight-speed modern automatic is faster changing gear.
The 599 is a world apart. Much wider, the car feels so much planted on the road, the limit seems sky high. But the engine is the predominant theme on this car. What an engine. Its simply bombastic. Amazing torque in any gear, at times way too much. The 550 and 575 can be driven at legal speeds as long as you are gentle with the throttle... the 599s engine is so fierce you simply cant. Its so powerful and brute you´re always driving faster than you think you are. And the cabin is so much comfortable and well isolated compared to its older sisters that doesn't help in this issue. So, wanted or not, the 599 is always running fast, with little notice from the driver seat or none even. And that´s maybe the real goal for the 599, its able to go so quickly with zero effort, in total normality from the drivers seat, it amuses. And then, pushed to the limit, boy it entertains. You really need to be prepared for such a beast, deploying 620HP with such torque is not for everybody. But its safe as past the limit its first reaction is understeer. Just be gentle with the throttle past the apex and the driver aids will help you make -lightning fast- progress. Turn the aids off and depends on you (gentle gas inputs and everything will go fine, be smart on the throttle and oversteer will appear, for fun or not, depends on the driver). Downsides? Size, its much wider, doesn't fell bigger behind the wheel but you do notice the lanes seems to be much narrower. That's the toll, size.
So, in my humild opinion, the 550 is a really fantastic GT. Comfortable, fast, driver focused under spirited driving, but safe in all terms and perfectly tamed when not pushed. The 575 is almost the same but with a spicier edge when driven fast, a little bit more capable but also that bit more demanding. And then comes the 599, another planet, a car around and engine (THE ENGINE, INDEED), much more capable in every term, lightning fast, easy to drive quickly but not tamed in any manner. You have to be aware of the speedo all the time if you want to preserve your license, and the country roads at times are too small for her... but when you return home and park her you have a smile for the rest of the day nobody will be capable of erasing it.
Live as if you´re going to die tomorrow, dream as if you´re going to live forever.