I've been using Sigma brushes for four years now and they've held up very well. I don't think they stand up to MAC brushes but the price difference is so significant that I will happily buy Sigma again. They're more comparable to the Sephora brushes but still a better price.
Back when I ordered the face kit, it was $39. Now, the same set is $57 but I think it's still a great deal. The only thing is that I ended up buying another contour brush because this one is so fluffy that it's really better as a blush brush.
Eventually, I'll move on to MAC brushes but if you're 19 (like I was) Sigma is great to start with.