There are 2 supply lines for the front sight.. SIG and Samson.. expensive bastards too. Several folks have changed out the POS 556 front end for the 551 forend that you can get from a supplier in Switzerland (if memory is serving me correctly) .. there are also know issues with standard AR rear BUIS being too high, most commonly noted in my research was the ARMS #40 series.
All reports have it as a kick butt and dead reliable weapon. The weight and balance complaint are very common with those who are primarily AR shooters. Those of us who have battle rifles (FAL, CETME, G3 ect) don't see the weight as bad as some do.
You did good skipping it if you were that concerned. They aren't cheap to trick out and personally should have come with sights out of the gate, kinda a rip off for a $1200 rifle.
First, I think it's great the original poster got the rifle they wanted. I do think that there are many "armchair quarterbacks" on the forums chiming in on the 556, no disrespect to the gent I quoted.
In reality, the gas piston design of the 556 immediately puts its AR competition into a very small group: POF, Bushmaster, DPMS, HK416, etc. Aside from personal ergonomic preferences, comparing them to S&W, RRA, etc. is unfair. The 556 is very favorably priced in relation to the gas piston rifles as most owner/operators will inevitably put many many extras on their rifle, especially optics and BUIS. Why should Sig add stuff that's just going to get replaced anyway and pass on that cost to us?
As for the dearth of accessories, it's simply not true. The 556 has the same picatinny rails that most ARs have, and can mount the same accessories. True, there's no readily available trigger upgrade, but get your hands on one and shoot it. My trigger is a thing of beauty, and needs no improvement.
True, there is a proprietary front sight dovetail, but Samson makes a very nice folding sight compatible with any AR rear BUIS (mine's a GG&G), and I'm certain it's only a matter of time before more are available. Aurora Industries has developed a very nice 551/2 style front post and matching diopter rear sight taken directly from the 551/2.
I don't post often, but as a 556 owner I felt compelled to respond. Lots of people erroneously compare the 556 to the non-piston ARs and even more unfairly the Sig 551/552.