I've used many methods to apply undereye concealer - fingers, flat foundation brush, wet RT sponge (both the big orange one and the small dark purple one, which are not exactly the same). This brush is by far the best. It applies my concealer smoothly and quickly. I just started using Shape Tape, and I really didn't like the way the damp sponges applied it. This brush totally solved the problem. So pleased! And for $3, you can't beat it. I had been looking at the Sephora Pro one, but I didn't want to pay $24 if I could get something just as good for $3.
Updated: I like this brush less now that I got the blurring one from IT Cosmetics for Ulta. The elf one is fine, but it's not particularly soft. It didn't bother me at first, but eventually I decided to try a softer one and I really prefer it. The elf one is good for $3 but nothing special for that price.