The lens is intended for low light shooting between f1.4 and f2.0. If you need Tilt Shift, buy the TS E 24 f2.8. If you are always shooting in bright sunlight, buy the EF 24 2.8 and keep the money that's left over.
If you have to have a low light lens with this focal length, it's the only game in town. On my 20D, it's more like a 38mm (fine for me since I prefer the equivalent 35mm focal length/angle of view on a FF camera). If I had a full frame camera, I'd go with the 35L.
Sharpness is very good at f1.4 if you have a little distance between you and your subject (benefits from increased depth of field). Up close, obtaining critical focus is essential since there is practically zero DOF at 2 Feet. Because of the limited DOF up close, you have to be careful if you want to obtain an acceptable result...else your subject will look unfocused.
I have been extremely pleased with photos taken at f1.8. I try not to go below 1/50s at f1.8 to maximize sharpness/contrast/color. If I need more light, I usually bump up the ISO.
By f2.5, the lens is extremely sharp (according to my eye).
By f5.6 or f8, color, sharpness, and quality is great as befits the "L" designation.
The bottom line is that this lens helps me gets results that are not obtainable with other lenses at this focal length. I have tried the Sigma 20/24 f1.8's... They are 2/3's a stop slower, and absolutely no comparison to this lens wide open.