Mixed, but I like unit trains or consistency in a few areas.
If it's a special purpose set like the Rocket Motor set, I would not likely mix that with anything else. Similarly for Tank Train sets with the interconnecting hoses.
I sort of have developed a thing for logging engines and cars. I wouldn't mix anything else in with the log cars, though I will definitely try to get different types (skeleton and conventional flat as examples) in the same train. I normally use the same road to match the engine, but I've picked up a few "orphans" for which I do not have a matching engine, so I'd probably sneak those in with another road name sometimes.
Passenger cars I tend to go for matching set pieces.
-Dave
