The Sigma 12-24 is a full frame lens. On a crop sensor camera like the 70D, it will have similar field of view as a
(12 x 1.6= ) 19.2 to
(24 x 1.6= ) 38.4 mm lens.
You might compare this to shots taken with other lenses near this full-frame length. Comparing this to an 18-55 might help, as you will get a sense of the middle range of the Sigma lens on the wide side. (18 is between 12 and 24).
In my experience, the widest that is really useful on a crop sensor is 10mm. Beyond that the distortion gets severe enough that it is difficult to correct in Photoshop. This is from personal experience, where I have found it quite difficult to control distortion in taking shots. Not impossible, just difficult. Of course, on a full frame camera, 9 and 10mm are often even more distorted.
Here are two examples, outdoor architectural shots, captured at 10mm with a Sigma 10-22mm, on a Canon 40D. They are of similar subjects, buildings in Rome. However, in the first, the camera is angled up, creating significant distortion. The second is camera level, which reduces distortion.
It is also important to compose carefully, as tall objects at the edge of the frame will be more noticeably distorted.
Therefore, IMHO, if you manage your composure correctly, your 12-24 will be quite useful.