My first paints when I started airbrushing were Humbrol acrylics, since I had used their enamels when I built models as a lad. I also tried Tamiya, based on what I read in fora like this one, and I still have quite a few of them. However, I was still experimenting with these (to me) new products, and Vallejo Model Air acrylics were next on the list. I built a Revell U-boat a while back, and bought the Revell acrylic paints the instructions specified. So, my thoughts are -
Tamiya are great, super quality and easy to work with, but require thinning with a solvent-based thinning agent. They also don't seem to go very far & need replacing frequently.
Humbrol acrylics are also good but, again, require thinning, this time with water. I haven't tried their new formula yet, which is sold in dropper bottles.
Revell acrylics are similar to Humbrol, but - as you point out - very thick, even sludgy. But, when thinned - with water - they work very well (the U-boat turned out fine). Trying solvent thinners with these is not a good idea - they turn into a horrible gel.
All these need thinning, and I don't like to do that in the airbrush cup as I can't properly judge the consistency, so I use disposable plastic cups for that, which is wasteful.
Vallejo Model Air paints spray without thinning, and perform very well (the metallics are very good). They are a bit fragile until properly cured, so care is needed in handling - I don't find this to be a problem. And, since no thinning is needed, I'm not buying plastic cups. So, until I try the new Humbrol acrylics, Vallejo Model Air is my favourite at the moment.
Hope this helps,
Hutch.