Here is a concept of a mech tank I designed nearly a year ago for a game called Vitalize that isn't released publicly as of this point. The enemies in the game were giant walking tanks that could be piloted remotely. I started to design the mechs in SketchUp as it proved more convenient to design such a complex structure in three dimensions. You could design your heart out with a pencil but then find that it doesn't work from another angle so 3D meant I was able to design from all sides simultaneously.
Above you can see the Photoshop paintover. The second image is the final in-game model where my SketchUp sculpt has been taken to the next level with more details and textures by Steve Walker. The rest are all renders of my original model.