I bought a cheap Tamron lens the other week, and its fine for what I want it for.
There's no way I could afford a top line lens so have to stick with the cheaper option.
These are the pics I got with it-
http://www.telsshells.com/kidderminster_r1.htm
Like clarky says, if you know that what your buying isn't amazing and you know that you get what you pay for then thats fine.
That Sigma is the equivalent to the tamron I bought so should give you a reasonable comparison.