Much better than music wire spring, not as good as tubbs - Strike Industries AR Carbine Flat Wire Spring

A lot of people think they'll get a precision spring at budget prices, but you can't have your cake and eat it too. This spring IS much, much better than a stock GI spring in both function and durability. It will last much longer, it'll also reduce the harmonic clash which causes your brass to eject in a random direction. But it's relatively loosely spec'd compared to a precision Tubb spring that costs $10 more, and that means it's not really any quieter than a music wire spring.

I have Tubb's flat springs in my precision rifles, and SI flat springs in my beater rifles. If noise is a big concern for you, spend the extra money and get a legitimate Tubb flat buffer spring. If you're only concerned with durability and consistency, the SI spring is fine. But use them side by side and there really isn't any comparison. The SI makes your rifle consistent, but the Tubb spring makes it run like a Glockenspiel.