I had an 0561 for a while, and it was nice and solid and smooth (after a brief break-in period), but it was a little unwieldy and carried kinda awkwardly - the area on the handle where the thumb studs rest when the knife is open is a bit sharp, and tended to rip holes in my pocket lining when stowing the knife (carrying it tip-up). I ultimately traded it for a Gayle Bradley.
Cut to a collection dump, a move, and about 6 months later, and I snagged a 0550BLK off the bay for a steal (seller knocked a huge amount off the price because it had no pocket clip, so I bought it and contacted ZT the same day for a clip and screws - which are FREE.) and it has been an absolute pleasure. Many people complain about the stock scale, but, while not being super attractive, it is very functional. The deployment is super smooth and the lockup is flawless. It arrived super sharp and S35VN is a wonderful steel.
Honestly, I like the 0550 more in practice. It's not as attractive (the 3D machined handle on the 0560/0561 is damned gorgeous) but it is every bit as functional. Also, the tip on the 0550 is a little more keen and is super stout.
If you're looking for a knife to use frequently, get the 0550. If you're looking for a knife to carry occasionally and admire, get the 0560/1. Not to say the 0560/1 isn't a functional knife, but the size makes it a little cumbersome.