I can buy AR-15 parts at the corner gas station if I ever need them.
Try that with a SIG!
You wouldn't need to.
"Worth" is in the eye of the beholder.
I own a semi CAR-15 clone and shoot M4A1s and M4LEs daily, so those bases were covered.
To me, it was worth it, but I got a pretty decent deal on a standard SIG 556. The fit and finish was quite a bit better than the standard Colt M4, military or commercial. I've encountered lots (20+) brand new M4s that had gritty triggers and selector switches, right out of the box. The SIG was smooth on both accounts.
Accuracy is similar, with the same ammo.
The SIG feels a bit smoother on recoil.
My biggest peeve with the SIG 556 is the lack of iron sights. The stock emergency sights are barely useable, even for emergency purposes. I used a Samson flip up front sight with a GG&G/Matech rear sight for a while, with good results, but the cheek weld left a bit to be desired. I finally broke down and purchased SIGs diopter rear sight and hooded front. This combination yeilded a sight height that fit the gun and allowed for a proper cheek weld, but it was expensive.
The optics that I've tried on it are the EOTech 552 and 3x magnifier, Acog TA47 (carry handle version) with flattop adapter, and the ELCan Specter DR. The ACOG is the only one that didn't interfere with manipulation of the charging handle. This combination also allowed the unimpeded use of the SIG diopter sights.