Bob Atkins provided an overview of these various lenses and the prices here . (It's interesting to note that in US, the Tokina is priced cheaper than Tamron, while in Singapore the reverse is true). Lots of user forums have commented about the Sigma 12-24mm (the oldest lens among the 4 in the market) and the Tokina. Even the to-be-released 10-20mm receives lots of buzz among the community. However, very little is mentioned about 11-18mm - no kudos, no brickbats. Here, I shall provide a simple review.

Here are some of the things that people usually ask when getting a third-party lens, and so I shall cover these points:
1) Specifications - Weight, Price
2) Optic quality
3) Focusing speed
4) Comparison with original manufacturer's lens

The Tamron 11-18mm is a light-weight lens - too light for some people. It's made of plastic with rubber linings along the zoom ring and focusing ring. In Singapore, this is the lowest-priced lens among the rest of the ultra-wide lens. 

In terms of optic quality, my eyes tell me that it is considerably good. The colours are rich and sharp, but exhibit slight fringing and distortion at the sides when viewed 100%. I believe this problem exists for all ultrawide and fish-eye lens. I am satisfied with the optic quality although the sides seemed soft and slight fringing when viewed at 100% (nowhere near Sony F828). The truth is: if you do regular full-size prints, you are probably a professional and therefore would not even consider this lens. For the rest of us, the effects are unnoticeable, especially after you resize for web view.

Also, bear in mind that ultra-wide lens will inevitably exhibit distortion depending o­n how you frame the subject. In the shot of the stairs (above), notice that there is virtually no distortion, while the building photo (right), distortion is apparent. Rule of thumb is: objects will be less distortion at horizontal eye-level. If you tilt the camera higher or lower, objects will be distorted. Here's an example of a low-perspective shot of the road (below).

For a scientific optic test result, I refer you to Bob Atkin's review here.

My first impression when I tried o­n the Tamron lens is: the focusing mechanism is slow and noisy! But focus is accurate even in low lighting, and I never did notice the noise when actually using it. Even the focusing speed did not hinder me when taking shots. The o­nly gripe is that the focusing ring rotates when focusing, so I had to be careful when I hold the lens. Due to the short length, I tend to rest my fingers o­n the focusing ring instead of the zoom ring.