I purchased the Face & Eye kit (w/ 6 brushes) a couple of months ago, and I have to say that I am quite pleased with them overall. While they are fairly comparable, they are far from identical to their MAC counterparts.
The SS187 is quite a good brush, but the brush hairs are of less refined quality compared to the MAC 187. It has much less resistance than the MAC 187 and is prone to splaying when doing the stippling motion. I find it to be less dense than the MAC 187 as well, but it could be the thinner quality of bristles that make up this brush. Because of this, I was very skeptical at first sight -- but surprisingly, it functions just as well as the MAC version and applies foundation and tinted moisturizer flawlessly. The brush head is also larger in circumference so it covers more surface and speeds up the foundation process.
The SS150 is EXQUISITE!! It is ridiculously full, dense, plushy and SOFT! I don't have the MAC version to compare it to, but compared to the couple of highend powder brushes I do own (Pout, Trucco) the Sigma 150 reigns. I've seen very minimal shedding too. A Beautiful, high quality brush and so cheap! I think this is my absolute favourite in the set. I highly recommend it if you're looking to buy a powder brush.
The SS182 is also a very good brush but I've experienced some significant shedding -- to be expected with dense kabuki style brushes, but I do hope the shedding stops soon. Wonderfully soft and dense and the brush hairs are of excellent quality. It's a bit smaller in circumference than the MAC 182, but it applies mineral/powder foundation just as nicely.
The SS219 is also a great crease brush. Compared to its MAC 219 counterpart, it is fluffier and more dome-shaped (whereas the 219 is pointed and shaped like a pencil). Very nice for precision blending in the crease. Again, the brush hairs on this brush are slightly less refined in quality/resistance than the MAC one, but still a very good brush.
The SS239 is a good brush -- I still like the MAC one more for its refined quality and precision though. The Sigma 239 came with a few long hairs that I needed to trim myself (same with the 219), so I get what some of you are saying about quality control. Nonetheless, it's still a great brush to use for packing on colour. Just a bit less refined than the MAC -- but for the price, I can't complain.
The SS224 is my second favourite and most-used of this set. I've been contemplating purchasing the MAC 224 for a few years now and have repeatedly marvelled over it at the MAC store. I'm happy to say I've decided against it. The Sigma 224 is smaller than MAC's and much more suitable for small eyes like mine. The brush hairs on this one have a good amount of resistance. It blends crease colours beautifully and I love using it to apply my highlight and to blend out harsh lines between the crease and browbone. An awesome brush and I get a lot of use out of it.
Overall, these are some excellent brushes for the price. Many of them are MAC comparables, but made with slightly less refined quality of brush hair. However, the lot of them function just as well as their MAC counterparts, so it's definitely great value for your money.