It's very good for a budget lens.
I have an older version of this lens, the SIgma 70-300 f/4-5.6 Super Macro II APO. It's *very* sharp, though can get soft above 200mm. Build quality seems fine, certainly not cheap. Bokeh is nice, and the macro mode is decent. Focus is a bit slow, but accurate on my Rebel XT/350D.
I've never used the 70-200L, so I can't personally compare them. But the Sigma is about 1/3 the price of the 70-200, so I'm not sure many people cross-shop these lenses. For the money, I think the Sigma is terrific -- better than Canon's 70-300 (non-IS) or Tamron's. If you want a bit of reach for not much money, go for it.