I have a 556 Swat Patrol (rails, shortened and with shorter gas piston). It is not my favorite gun, but I like it. When I first got it, I had an issue with one part. After a couple of trips back to Sig, it runs like a top. It's a lot faster to clean, too, than a DI AR, since the only hard carbon fouling is on the piston head and regulator, and they're designed to run OK when dirty. The bolt group will get stuff on it, to be sure, but it's soft soot that wipes off easily, not the hardened gunk that ends up on DI guns and has to be scraped off. Some people have had rail issues, but mine are straight and lock up pretty tightly.
The other posters are correct that the number of aftermarket parts is very low. Troy has a rail system, though I think it only works on the long-gas-system versions (not "patrols"). Manticore Arms has a new grip in the works. There is a company in Colorado that imports the Swiss furniture found on the rest of the 55x series, but it costs a pretty penny. PMAGs, etc., work great. The folding stock is cool, if you like that sort of thing.
Sig has had some QC problems with the line. Once/if you get an in-spec rifle (either OOB, or with a warranty return, which they handle well and quickly), however, it's an incredibly robust gun that feeds and ejects reliably.