Ive been testing this lens now for about 3 weeks and I have to say I am very impressed. My other Macro was a Canon 100mm 2.8, but it was also much bigger. The Sigma, is smaller, has a minimum f stop of f 45 and allows for a 1:1 magnification ratio. The result is some very impressive (and BIG sized subject) macros.
My recommendation on it is this, if you do not have a good macro, take a close look at it. It is much cheaper ($265) than a Canon 100mm 2.8 ($450+), and for what I use it for, I think it is as good as the Canon. This is currently the only non Canon Lens I use regularly, but quality images are all that matter in the end, and for macros this little guy is amazing.
Sigma 50mm Macro