I don't have a 555, but I have ridden one more than once. Fabulous frame. What are you comparing it to? What kind of riding do you want to use it for?
It's relatively stiff - on par with my Giant, though not as stiff as some other carbon frames I've tried from Orbea, Merckx, Specialized, etc. and definitely less stiff than the Ridleys. I don't race, so I found the characteristics of the 555 and the TCR to suit me very well. Good power transfer, but comfortable on those centuries. The 555 is a slightly on the heavy side, though Look makes up for it with the incredibly light HSC5 fork.
I narrowed my choices down to the TCR and the 555. I went with the TCR, but verall, I say the 555 is just as great a value for a high-modulus carbon fiber frame.