I had one.
The gun was accurate as all get out when shooting from a rest, but difficult to shoot very accurately off hand without a shoulder stock. I tried the sling method to help stabilize it, and it worked alright, but was still far behind a full sized rifle for practicality purposes. I put a Troy rail system on mine, and a laser, and bought pmags, and Troy BUIS.
All in all, it was a fun gun, but with the shorter bbl, the lack of a stock and thus stability, the potential use for the gun was severely limited to range toy only.
If you are looking for a defense weapon, you'd be much better off with a full sized Sig556 or AR. If you're just looking for a toy, then have at it.
Ultimately, I had put about 500 rounds through mine, and just couldn't warm up to it. I ended up trading it for a S&W 686+ and a Ruger SP101 in .22 lr.
I realized I just don't care about AR type or military guns. I also found that no matter what I did, I degenerated to trying to really pepper the target fast with that gun in some sort of Rambo fantasy, and really just shot up a lot of ammo very fast. I do better with revolvers anyway and regular old .45 acp's.
Another note, if you do not use a sling, the P556 takes a bit of upper body strength to hold up and keep stable while shooting. I found it a little awkward myself. I could shoot it fine, and fast, but I believe for the effective range of the gun, in a defensive situation, you'd be better off with a handgun caliber carbine. JMO If you have more specific questions, ask.