I picked up 4 bottles of MM Paints a while back and used them on a build and was impressed how well they sprayed. I followed the directions but the expensive Poly additive went bad in the bottle turning into jelly being about 2 years old. I contacted them asking for a replacement and was told I was SOL. Some of my Testors enamel paints are 30+ years old and are still good.
I did add a few more drops of thinner than recommended and also noticed a good solvent primer is needed otherwise the paint will easily scratch over bare plastic.
I do not plan on getting any more of their paints and rather use the lacquer based ones.