I have used Fray Chek for a long, long time, beginning with using it to secure cross stitching on tea towels (which then went through the washer and dryer a million times...the towels fell apart before the stitching!). I often use it now on edges of fabric before laundering and pressing, on points in stuffed toys that I think might fray, ie. the beak point of a stuffed bird, or anywhere I think a seam will be under huge stress and might degrade.
I like Fray Chek. It does feel odd and a bit stiff at first but quickly "works in" and becomes invisible and sufficiently soft.
That said, I will give Fray Block a try next time I have to restock, and see how I like that product!