The Tokina is great, I have it, many of my best landscapes and architecture shots are taken with the tokina 116. I have also taken a few shots of the stars, long exposures at f2.8 and they come out sharp, it is incredible to capture the milky way . At 11mm is hard to get something out of focus, i have taken a few shots guessing at the focus distance, or letting the camera pick the focus point and they come out great. i found that 1/30 is about the slowest i can shoot handheld without blurring out the image, at 11mm it gets sensitive to motion blur at the corners, composition gets tricky it is easy to include distractions on the frame the minimum focus distance of 30cm is also great, allows for very dramatic shots. Auto Focus speed is fast and precise. The only disadvantage i have found is that portraits with this lens do not come out nice, specially if you shoot a landscape and want to include a person on the side of the frame, perspective on the sides will make them look fat, another oddity but skin tones and shades on faces come out looking wrong. perhaps it is my technique, portraits are not my forte.

I also have a 18-200 zoom, and often found myself wanting wider

Sigma has an even wider lens, it goes down to 8mm, but i have no experience with that one. Max Aperture i think is F4 , and i think it is more expensive than the tokina, the additional 3mm will be certainly noticeable at the wide side.