Personally, I find the thickness of the paint hinges on the color, i.e. the amount of pigment in the paint. I find lighter colors require a lot less thinner than darker colors, maybe that's just me. I personally like the dropper bottles for adding to my airbrush color cup, they're a lot easier to use than an open lipped jar. Most of the time, for lighter colors, I use only a drop of thinner, for darker colors I mix almost one to one thinners. I find their coverage very good.
I dislike their primers quite a bit, so I've swapped over to AK Interactive white primer, I find it a good coverage and it stands up to remedial surface work. I found the Vallejo primer to be like a layer of rubber, and I could not sand or file it at all, it also lifted quite easily if masked.
I both airbrush and hand brush the model air line, with no difficulty. I've played a bit with their mediums for transparency, often with good results. I also quite like their matte and satin varnishes. I dislike acrylic glosses in general, I use lacquers. I had to do a little work to track down their proper RLM 75, but I generally like their colors and find them reasonably accurate. I've used this brand of paint for about 2 years now, and I'm pretty certain I'll stick with it.