Recently, my LQS switched to selling Aurifil thread. I had to buy a few spools in colors to match the applique project I'm working on. Wow, is this stuff ever fantastic! I am using the 50 wt. size which comes on an orange spool. It is quite fine and disappears into the applique shape, even with my beginner appliquer less-than-perfect stitches. I want to try it for piecing as I suspect it will make for flatter seams and help increase size accuracy.
Before trying Aurifil, I did not think thread could make much difference in the final result. Now I am a believer!
P.S. The Aurifil thread company doesn't know who I am and did not solicit this post.