I traded an 'L' lens in a similar focal length for a 300 f/4 and I wanted something cheap to backfill this range. Found this Sigma on Craigslist.
Reviews said it was loud and heavy and the AF was slow. Made sure I pointed that out to the seller. He knocked $50 off an already-fair price.
Got it home and started using it. Loud, heavy, slow? Compared to what?
If USM is silent, the Sigma AF is all but silent. I'm no doctor (I just play one on the internet), but if you find this objectionable, you might consider medical attention. The sound of a shutter firing may trigger an aneurysm.
It weighs about the same as a Bud Tallboy -- which is always welcome and comfortable in my hand.
With AF speed, faster is always better. But unless you're a Paparazzo after that quick, sharp glimpse of JLo's underthings, the AF here, on a lens of this FL, should be more than adequate.
I've yet to shoot any test charts or parts of $5 bills with this lens so I can't comment on that. But I do own some lenses that, when I pass prints around, I have to watch the kids so they don't poke their eyes with the sharp images. This isn't one of those lenses.
I have found, though, that I've used it more than I thought I would for shooting actual things -- like you'd find in the wild. For that kind of work, considering the price, it's a pretty nice lens. I'd recommend it.