I have a Patrol 556 in 5.56mm. and a 556R, which is 7.62x39 and uses AK-47 magazines.
These are nice reliable guns for the most part. Early ones had some teething problems and the 556R has had some revisions for durability in the magwell. Early problerms were mainly canted rails, and partial separation of the rail, which are not integral to the receiver as on the AR-15 rifles & carbines.
These tend to be front heavy guns in comparison to the AR type. They use pistons and are "inspired" byt the Kalashnikov design and this is very apparant in the bolt and bolt carrier design. A big difference is the piston disconnects in these guns and the charging handle doubles as the lock key for this purpose. The piston tube is removeable and the rifle itself breaks down easily for cleaning.
Drawbacks;
There aren't as many accessories available for the Sigs as there are for the Ar - 15 style.
The SWAT type Sigs have a continuous weaver across the top so BUIS, front & back, can be used from the AR catalogs, but if the style of the Sig only has the rail on the receiver and a front sight in a groove on the gas block you need a special type of front sight to get the height correct. Midland Industries makes one. You have to be careful though because not just any AR front sight will work.
If you get a newly made one you stand a good chance of getting a nice rifle that will probably be quit reliable. They are not *AS* accurate as the AR type but are still very good.
One nice thing is the Sig has an adjustable gas system so even if you do get it really dirty a flip of the switch allows more gas to operate the piston and it will probably keep you going until cleaning time.
Red dot sights will work, Eotechs will work, that sort of stuff works, various slings will work fine. It's just those BUIS front sights that are tricky.
Any AR accessory that is special to the AR (like handguards, shoulder stocks, etc., will not fit the Sig. That to me is the one main drawback; the relative lack of accessories for the Sig.