I got back into modeling recently after 30 years, and Humbrol paints were recommended to me by my local Hobby Shop. I am used to using Testors paints and only work in enamels. I've used two of the Humbrol tins of paints that I purchased and so-far both have had thick wads of paint in the bottom of them that I've had to dig out with a scraper! I've successfully re-thinned them in a glass one ounce paint jar, but it's been a pain in the behind. No such issue with my Testor paints. I just bought a bottle of Tamiya and we'll see how that goes soon. I'm feeling like I'm sticking with Testors unless this changes.
P.S. And yes, I've shaken both tins very very well before attempting to use. It's like thick MUD in there still however. More like CLAY really. I'm giving Humbrol one more tin, but after three strikes, they're OUT! The other three tins that I purchased will get tossed out if that occurs.
Errata: Wurger, you mentioned proper preparation. With my Testors I just pour it in the glass spray-gun bottle in a 1:1 ratio and it's good to go! With the Humbrol's so far, it's been a lot more work--at a 1 part paint/2 parts thinner ratio--and I still don't think it's spraying quite as well. In the end, I'm going to go with the paint that is the least hassle to use. I still have to try Tamyia however. I'm not sure why some say Testors isn't that good. I've never had an issue with them in 40 years of using them. (Well, in the 80's, a one-off model in the 90's, and now again)