For what it's worth, I got the Sigma 18-200 last year and used it all season. I was in the same boat - seeking flexibility to avoid in-the-field lens swaps. I knew what I was in for, but I still ended up being a little disappointed by the image clarity. What you gain in flexibility, you definitely give up in crispness. Shooting f6.3 in the 50mm range gives very nice results, but at wide aperture near 18mm or 200mm, there's a subtle muddiness to the images (not necessarily just at the corners either). And really wide or really tight is how I bet most chasers would shoot (definitely how I roll). That said, the Sigma really is very reasonable price for what you get, and I'm probably just extra picky.
I'm still not sure what I'm gonna do this year in the lens department. Renting some primes is definitely tempting, at least for a few chases.