I have a Canon EOS 30D that I've owned for about 2 years. I've been wanting a macro lens for a while, but have had a hard time justifying the cost... especially since I originally had my eye on Canon's 100mm macro. Then someone suggested this Sigma lens to me, and I noticed that all the reviews were incredible. I was nervous about going with a 3rd party lens, but the price was right and the reviews gave me confidence. And also, I preferred the 50mm focal point vs Canon's 100mm (makes it a more versatile lens, in my opinion).
So then this christmas my dad gave me money to buy the lens, and I've only had it a few days now but I am already in love! This lens is wonderful-- the look and feel are good, and it takes GORGEOUS photos both close-up and otherwise. I've simply been using manual focus for close-up shots, but I've been very surprised at how fast the AF is (with the limitor switch on). It really is almost instantaneous, which is awesome. I think even faster than my other lenses (Canon 50mm f/1,8, Canon 17-85mm).
I borrowed my friend's 100mm macro lens last year, and honestly I don't see a significant quality difference between the two (though I am also not a pro, so may not have a critical enough eye for that sort of thing). Basically, I am VERY happy with this lens, right out of the box. The photos are gorgeous, razor sharp at the focus points, beautiful bokeh elsewhere, colors are very pretty, etc etc etc. This is a great alternative to more expensive macros. I actually wonder if this will replace my "nifty fifty" as my favorite lens... we'll see. ; )