Does anyone have experience with this lens? I currently have a Sigma 18-200mm f/3.5-6.3 on my Fuji S2 Pro. I find my current lens isn't exactly as sharp as I would like it to be and it's too slow (not to mention it feels really cheap). I've always loved using the Nikon 17-55mm at work but I'm wondering if there is any reason I should not spend $1200 on one?