For an off the shelf ar-15, the sig is ok, but it is a bit pricey. Not as bad as some, but more than you need to spend for a good M4.

One of the biggest advantages of an AR platform is perhaps it's modularity, along with it's popularity. What this means is the platform lends itself to easy customization. You can start with a bare-bones model, or even just a list of parts, and put it together (or find someone to do it for you for a modest price), for less than $700. Now that is a lot more than many good bolt action guns, but an AR is capable of a lot more than a standard bolt gun shooting the same or similar cartridge. While it is correct to say a good AR will do everything a bolt action can do, the reverse is not true. In certain applications, this difference can be extremely important. This isn't disparaging of bolt guns. They just have fewer applications.

Back to the modularity thing. Since AR's are component guns, it is fairly easy and relatively common for AR owners to modify their guns in various ways. In my case, I have an AR that I can swap out my barrel and forearm (referred to as the upper) just pulling two pins out of the receiver. I can then swap between a short barreled carbine with a compact hunting scope mounted on it, which I sling for expeditionary work, or a long heavy barrel with a bipod attached and a large, heavy, high powered target scope. I can also swap out uppers and change cartridge capabilities, or I can swap out just the bolt and magazine and shoot 22 from the same gun.

There's more advantages to having/using an AR, but these are the bigger ones to me. Of course, I like having a good semi-auto that I put together myself and will unload 30 rounds at 300-400 yards in a minute and bang the gong on every shot. But that's just me.