most advanced photographers say that are about 50/50 happy with non-Nikon 18-2xx lenses. I had very bad experience with Tamrons, but Nikon 18-200 VR or VRII it's just point and shot - sharp enough every time. I hate Tamron's VC system. I suggest you to try one of the lenses you are planning to buy used.
here is a snapshot of my daughter taken tonight with D5100 and original Nikon 18-200 @24mm , ISO 800, 1/40s, f/5.0