I've used them for buying coloured self adhesive vinyl for logos and decals and the vinyl seems pretty good but in all honesty, I've not tried other sources so I have nothing to really compare them with. I want to try the cool-flex and Hot-flex materials next for some Tee-shirt logos but as with most things, the great ideas alwasy seem to end up sidelined due to a lack of time
For airbrush masks however , I use either the Art-tool Ultra Mask which I bought on a reel many moons ago from
here or more recently, I've started using the TrickBits Maskit Masking Film from Horizon Hobby UK stockists (Part Number TB5700). This seems to cut nicer than the Ultra Mask and is a translucent blue colour so it stands out a bit more than the grey of the Ultra Mask.
HTH