Has anyone who has used this lens talk about its sharpness? I would like to know how this lens compares with the 18-200mm DX lens? I use a D90, and this lens is intriguing for travel purposes because of its range. I would think that coupling this with a 12-24mm and a fast prime would make the perfect vacation kit for DX shooters, but I'm concerned about the softness and lens creep issues that plague the 18-200mm. Any thoughts on this?