I have 2, an SBR and a 16”. I think they are great, with a few caveats.
Cold hammer forged Nitride treated barrel, 1:7 twist (I think the DMR type has a longer twist). Both of mine have excellent triggers. The ancestry and derivation are from a sound rifle (SG 55x) with a great BCG design and a robust and proven operating system.
The ability to take STANAG magazines and a familiar control layout were benefits to me as I have several ARs.
My 16” has been surprisingly accurate, and the SBR just average but still quite acceptable. Both have been absolutely reliable with no malfunctions over about 1k rounds, and they by nature run very clean.
Now on to the caveats.
There are a few versions of the 556 series. The early guns with the AR type fixed or folding/collapsing stock setup and, the later “classic” series with the Swiss 55x type folding/collapsing stock setup. The lower receivers are different in that respect as one has a threaded receptacle for an AR type receiver extension and the other is made to accept the Swiss type stock, so it’s one way or the other- no changing between. In my opinion (shared by a great many others) the AR type folding setup is just plain junk. If you have that type, just mount a mil-spec receiver extension and the stock of your choice. If you want a folder, go with an ACE AR type setup, it’s fairly robust. But, either way get rid of the junk Sig folding mechanism. The “classic” Swiss type folder is fine. There have been some who express concern over the steel latch against aluminum receiver possibly leading to premature wear, but I haven’t seen a problem in use, nor have I heard of any actual problems. The early “classic” lowers have a Swiss made stock and (like the early standard uppers) were reported to have many Swiss internals. Later examples no longer have the Swiss made stock but one that looks the same, feels slightly different, and is of unknown origin. I am not aware of any problems with the later stocks but some say they feel cheaper…
The rest of the furniture- The early guns had some funky polymer hand guards that were unnecessarily heavy on an already front heavy gun. They were also funny looking. 
But, they were robust and had small rail sections. The later guns had the “classic” Swiss style hand guards that are much better, light, and also accept small rail sections. The only complaint I know of on those is that they can be a bit loose at the front. BTW you can fit the “classic” type guards in place of the older fish gills. For rails the Sig rail is fine, Samson has a nice rail, as well as Troy (free float I think) and possibly some others.
The 556 top rail is fixed to the receiver with screws as opposed to welded on like the Swiss 55x series. Some of the early examples had canted rails and Sig would often swap out the whole rifle or upper on return. Some have reported the rail screws loosening on their rifles. Most report that upon degreasing and lock tight application it doesn’t reoccur. I don’t know why Sig went this way, I wish they didn’t but, I haven’t had any problems with either of mine. This does allow the aftermarket rails to have a “monolithic” top rail as they replace the whole thing.
Sights/Optics- There are several versions of sights on the 556 series. Firstly let’s have it said that the Sig red dot Aimpoint T-1 knockoff is a disgrace for a company like Sig and I don’t know how they get away with it. I sold mine on TOS, there are plenty of buyers… Now the BUIS- It should be said that in the case of the 556, BUIS stands for Buyer Understands I Suck. The popsicle stick rear is delicate and has an imprecise adjusting mechanism. But, I was able to get surprisingly small groups with it. The front sights are fine for the most part. The issue with replacement is that the 556 has a lower comb height/cheek weld than an AR and thus lower profile sights. So if you throw on an AR rear BUIS it will be higher and also require replacing the front with a matching height. There are several quality options out there if you are OK with the higher cheek weld of the AR sights, including folding fronts that fit in the Sig front sight gas block dovetail if you don’t have rails. There is also the micro version of the excellent Troy folder which has a matching rail mount front and also lines up with one of the assorted Sig or Swiss SAN gas block front sights. I have tried out the Sig diopter set on the SBR and am un-impressed. The front seems fine but the rear is less than confidence inspiring and took an awful lot of elevation to zero. I will be going with either the Troy micro folder or one of the Swiss options. For the Swiss options there are some great sights but, they are a bit hard to track down, and they are expensive, worse than buying KAC stuff. 
Anyway, there are quality sight solutions out there. Whatever one does with regard to BUIS just be sure to pay attention to the various heights and match them up.
Magazines- The Sig magazines are crap, sell them, people want them… PMags work fine as well as any quality AR mag.
As mentioned some people with small hands have trouble reaching the mag release or safety without shifting their grip. I have large hands and no problems. But the large KAC mag button will fit and fix that part for those who do.
Overall I think that if you square away the folding stock and BUIS it’s a fine gun. No problems with either of mine. Out of the box a “classic” type should be fine but for the sights. I wanted an SBR so I bought a “pistol” and registered it. When the stamp came back I bought a “classic” 16”. I swapped the lowers for an instant SBR. I then put a Magpul UBR on the 16” for a nicely balanced setup.
There is a large population who will never like it and are upset with Sig because it departs from the Swiss 55x series and I get that. They want and have been waiting for a “US 55x”. I don’t know that it would have sold nearly as well without STANAG mag compatibility. The early quality control issues and cheap furniture/accessories also undermined the credibility of the system and their reputation.