I couldn't get the hang of Sketchup to be able to tell you if it's any good or not but I have also heard it's good XD I use Blender. It is extremely useful for that purpose, I usually use my own characters as bases when drawing (because I can't actually draw) and also have mocked up an entire animation base for my sibling (who actually does 2d anim) because it was a rather complicated first person one with very drastic angle changes. Definitely speeds things up XD