I had bad early experiences with Humbrol paint. My usual method of mixing paint is to stick two BB's into a bottle or tin and shake vigorously. This didn't work on some Humbrol paint tins. So I started putting two BBs into a new tin and using a toothpick to stir up the "gunk" that has settled to the bottom. Then I close the tin with a rubber mallet and shake vigorously. The paint comes out really well after that. I suspect that people who have bad experiences with Humbrol paint (and possibly other brands) are simply not mixing the paint well enough.
Martin