Most of the complaints about the Sigma 150-500, Tyler, concerned a batch of early lenses which had auto focus problems. I did have the batch numbers but I'm not sure what I did with it.
I haven't heard of any problems recently.
My lens certainly works well, right up to 500 mm; but subject to the comments above about slightly slow autofocus and needing reasonable light to shoot at the optimum apertures which are between F8 and F11.
I also have a Sigma 180 macro lens which gives excellent results.
Most of the complaints about Sigma quality, as far as I can see, refer to their budget lenses which are produced down to a price instead of quality being paramount. And these problems are likely to occur with any ultra cheap lens.
A little while ago, I purchased a secondhand camera which came with a Sigma 28-300 lens which, in my opinion, wasn't fit to be used as a doorstop! But what can you expect at that price.