I have used army painter alot, since it being a danish product at all, and had fine experince with it overall. Painted a thousonds army in a matter of hours do to their metalic spray.
here are some of the stuff I have heard and seen.
1. Just to started with the daemonic yellow being mentioned by Talos. You have to do a thick layer to cover the model and get a real yellow colour. If you want to spray yellow I would suggest first priming white and then using a airbrush.
2. One of my friends say he prefere GW because he think its thiner. Here we are only talking about black and white.
3. Crusting - I dont know how I should describe this best. Many danish painters and gamers seems to have experinced the army painter spray crusting, giving a gritty and ruff undercoat. In short terms it ruines your model. After what Im told its a matter of keeping the can at the right temperature, and making a short trial spraying before spraying.
This is just what I know and think.
"Baby, Im a heretic, and your a spineless imperalist"