The 0550 is my favorite ZT folder!
I have two the 0550 and the 0550BLK. The former needed a little break in since the titanium lock was sticking (sharpie treatment plus a couple of hundred open/close cycles solved it in a couple of days). The latter was perfect from day one.
Opening the knife requires the thumb to be placed at the six o'clock of the stud and pushed straight up and then clockwise. Once you get the hang of it it's silky smooth.
On my 0550BLK I put a Rick Henderer Carbon Fiber scale and not only does it make it look stunning but it feels better in my hands with a much smoother texture and narrower.