Also, what's your opinion on 2d art placed over 3d backgrounds, like say this...
Works pretty well when you adhere to a common theme and don't just throw everything and the kitchen sink onto the canvas.
Pat's got it right. The simplistic coloring and texture of any 3D object can blend well with 2D sprites. If you think about it, the 3D objects basically have 2D textures applied to them, especially if it's from the same artist or two artists with similar styles.
There's a hierarchy of character design that best explains this. One side of the spectrum you have "iconic" cartoon characters like Hello Kitty and 1920's Mickey Mouse. One the other side you have "realistic" characters like from comic books or Shrek characters. In between you have Simplistic (Hanah-Barbara, PowerPuff Girls), Broad (Tex Avery toons, Roger Rabbit), Comedy Relief (Kronk, Mushu), and Lead Character (Cinderella, Belle, Almost any Disney Princess that is one level below Realistic). You wouldn't put together an Iconic-Styled character with a Lead Character in the same setting together if you were aiming to make a believable film or game. But narrow the spectrum and put a Comedy Relief character with a Lead character in the same world and it makes sense.
As in the case with games, you wouldn't use those 2D sprites and put them in a game that has super-realistic textures and 3D backgrounds similar to Modern Warfare games or Crysis. As long as the artistic style stays within a narrow spectrum of simplistic or realistic, you can do whatever you like with blending 2D or 3D.
What do you think is a better way to make highlights and shadows, brushes or lineart?
I find that every artist has their own method of doing things, and there is no real right-or-wrong way of doing things. Like any simple, cheap ball point pen, the quality of the tool doesn't matter, but the skill of the artist that uses it does. You find what works best for you. And there is nothing wrong with trying out things that work with others. If it works for you too, Run along with it. If it doesn't, dump it and move on to something else. How well you use any tool in your craft will provide good work on it's own if you practice. Talent has nothing to do with it. The word "talent" is just other excuse other people use to excuse themselves for not trying. Talented people are talented because they practiced, developed a skill, and didn't stop doing what they were doing. I've seen people do amazing things with the simplest of tools like MS Paint. I can't do what I like to do with that program, but others can. You use the tools you are accustomed to and grow, expand, and experiment with it.