When it comes to cost vs benefit the Sigma 15-30 delivers. I doubt you can find better performance for the cost. Its somewhat soft in the corners shooting with some (easily correctable) chromatic aberration when shooting wide open at f/3.5 but from about f/7.1 onwards it provides sharp pictures with its best corner performance on full-frame cameras from f/9-22. The reviews show this lens sharper than its 12-24mm brother and my 15-30 is much sharper than my Canon 17-35 f/2.8 L.

Its 15mm gives a wonderful ultra wide angle. However, its weakness is flare – especially at 15mm, where any sunlight that hits its protruding front element results in flare.

Here are some representative samples from f/4.5–18 as well as two examples of lens flare - all taken from full-frame cameras.