Let's get this out of the way - I paid for all of these myself, I have no coupon code for you, and Sigma has no idea who I am. I know I always get suspicious reading reviews of products made by companies that engage in the YouTube / Instagram blitz, so I wanted to get info out and up front. I own 5 Sigma brushes: the E25 blending brush, E50 large fluff, E65 small angle, E15 flat definer, and F70 concealer. Each of these brushes are similarly priced, coming in around $14-16. My oldest Sigma brush is about two years old and the newest has been in my collection for about 9 months.
Sigma's E25 is their riff on the classic MAC 217 blending brush - a fluffy blending brush with an oval footprint. This shape of brush is classic for a reason. I use this type of brush for laying down and blending eyeshadow, but some also use it for concealer or primer. I would advise against buying the Sigma E25 and going with the Hakuhodo J5523, which costs an extra $5. The Hakuhodo is superior in every way - softer, higher quality materials, better brush head shaping, and retains its shape over time. Everyone needs a blending brush in their collection, but once you are prepared to spend $14, you might as well spend $19 for a brush that will be a pleasure to use.
E50 Large Fluff
I love large eyeshadow brushes that are great for a sweep of base shadow over the lids. I was excited to try out Sigma's version after seeing some positive reviews, but this brush is a dud. I personally love the shape and size, but the quality is a real let down. The taper at the top of the brush betrays inferior material quality and brush head cutting. I find the bristles to be pokey and scratchy at the tip, so it can't be used comfortably when perpendicular to the eye. I use it rarely and only use the paddle shaped portion of the brush parallel to my lids to 'make it work.' The more affordable Ecotools large eye brush, while smaller, has a smoother taper and dramatically softer bristles.
E65 Small Angle
I actually like this brush and find that the shape and size work really well for me. I primarily use this brush for eyeliner. My one complaint is that the angle shape has not held up as well as I would have liked. After more than a year of regular use, the shape has lost some precision. Part of that is my fault because I am often lazy and use the brush when it is dirty and slightly stiff with eyeliner, but my elf studio small angled brushes are a quarter of the price.
E15 Flat Definer
I thought this brush would be good for tightlining and boy was I wrong. This brush is absolutely too wide to tightline and always leaves me with odd speckles of product on the outsides of my lids as a result. I tried to make it work by using it with eye primer to spread the product over my lids, but I was only marginally successful because the bristles are too short and narrow to do the job right. The brush isn't bad quality, but the shape isn't useful to me. I would recommend the Hakuhodo J521, which is the same price, superior quality, and the right shape to get the job done.
If this brush had been executed correctly, I think I would really enjoy it. Unfortunately, this brush has aged terribly, betraying serious quality issues. Over a matter of months, my bristles have splayed significantly and the shape of the brush head has begun to lean to one side. On a production note, the handle of my F70 also bows in the middle, which is frustrating. The hairs have splayed so badly I had to trim some errant bristles that were interfering with my concealer placement. The bristles on this brush are soft, the size works for under eye concealer, and the concept of the brush head is solid, but the execution is appallingly bad.
Overall, I just can't recommend Sigma. Sigma brushes don't age well and are inferior quality for the price you pay. Sigma brushes are about double the price of Ecotools and quadruple the price the elf studio line and not any better in quality to show for it. If you want to pay Sigma-esque prices, I would recommend Hakuhodo instead - their quality is better in both materials and execution.