Make sure you check out my standalone review for Sigma Brushes, which considers them on their merit and by themselves, not as an alternative to MAC specifically.
The majority of brushes are not on the same level as MAC, and there are some subtle differences that translate in application that makes me think Sigma really would be better off having their own numbering system and designing original brushes and improving upon their existing designs or similar-to-MAC brushes. Though the numbering system has made me hesitant to review Sigma since I first heard of the brand (because I don’t like copycats, diversion, knockoffs, etc.), I think they could do just as well without the MAC numbers. I think it automatically invites very tight, very critical comparisons between the two brands.
Sigma Makeup brushes are definitely a more affordable alternative to MAC brushes or any other high-end brushes. They are good, but not always great and sometimes just so-so. Like all brush ranges, not all brushes are made equal. Some brushes aren’t as soft, others not as useful. I have all of the currently available MAC brushes, and there are certainly brushes that I have no use for and some that I don’t love or even like. To expect every Sigma Makeup Brush to be outstanding or to surpass MAC or other high-end brands is a very tough expectation to meet.
I think if you go in with “this is an affordable alternative” rather than “this is exactly the same or better” mindset, you won’t be disappointed. (Of course, also make sure you’ve read my thoughts on the brushes themselves, because some might still disappoint you–e.g. 187!)