The Sigma 20-200mm sounds like a perfect do it all zoom lens when in good lighting. I keep getting tempted to pick it up. For example, it sounds like a perfect lens to take when I go fishing, since light isn't an issue and changing lenses can be challenging. The 20-200mm range would also cover most of my non wildlife photography needs.
But for times when you know the light is going to fall, part of me feels like it's going to end up becoming another lens in my bag instead of making my bag lighter. After spending a day in NYC recently, I am wondering if my best move might just be a second body. Swapping cameras should be quicker and easier than swapping lenses, the weight would be about the same as adding an extra lens, and having a backup camera with you is never a bad thing.