I really like them, I'm starting to convert my entire paint supply to them in fact. I like their ease of use, but one caveat--use their thinner. They dry fairly quickly, and their thinner has some retarders in it to help. I've also used Liquitex's slo-dri as a retarder with good results.
I keep G.S.'s thinner and slo-dri in eyedropper bottles (that used to contain cleaning agents for my contacts--clean 'em well!), it's really handy to dispense a few drops at a time into my airbrushes color cup.
Dan