These are the two brushes I got from the Semi Precious collection. A lot of people were calling them "gimmicky" and just a way for mac to make more money. While I don't think that they're "gimmicky", I also don't think that they're must have brushes.
I think people think you have to use one side at a time for these brushes, which simply isn't true. You can use the light side, the dark side, or both all at once. That's the beauty of this brush, there are options.
The way I went about testing these brushes was by doing a challenge where I had to apply all of my makeup (not including brows, lips, or liner) with just these two brushes. Here's how it went:
I started out by using the synthetic side (the beige side) of the 234 brush to apply concealer to my lids. Flipped it over to the brown side to blend. By blending, I used primarily the brown side, but had it angled so that part of the lighter fibers would help it blend as well. To go on, I used the synthetic side to apply and place the eyeshadows, while using the other side to blend them.
For my face, I used the 128 brush almost perpendicular to my face, with mostly the brown fibers touching my face, to apply my foundation (studio fix fluid) in a circular buffing motion. I have to say, it applied it flawlessly. It applied it similarly to the 109, except no shedding. It gave an absolute beautiful finish to an otherwise very thick foundation. Then I took the synthetic side, and applied a cream blush. Again, it applied it just beautifully. Actually, this is the best brush I've come across to apply cream foundation. You can apply the cream with the light side, then flip it over to blend it a little better. Then I used both sides of the brush to apply a finishing powder.
I will say, at a separate time I tried applying face and body foundation with the 128 and it completely lumped together the bristles and made it feel gummy. Also at a separate time, I used it to apply regular blush and it applied it just fine.
So all in all, if I had to only have one, it would definitely be the 128. It really enhanced the way the foundation and blush looked. The 234 is just OK for me, I can take it or leave it.
Hope that helps some of you!