I own the Sigma DC 55-200 and I love it. It's an underrated unknown gem for the price. It is surprisingly sharp all the way and even wide open.
Back when I bought it I also considered the DA 50-200 and Sigma/Tamron 70-300s.
Based on reviews and forum posts, it seemed to me that the 55-200 had the best IQ. The DA 50-200 was often reported as being soft off-center and especially wide-open. Someone in a Canon forum compared the DC to a Canon equivalent lens, with favorable results for the Sigma. On top of that, the Sigma was considerably cheaper than the competition - dirt cheap at $150 (vs $220 for the DA at the time).
Like I said, I am very satisfied. I get very good results, convenient zoom range, surprisingly good macros at 200mm with a good diopter, and in a light and compact form.
The DC 55-200 is in fact discontinued, replaced by the DC 50-200 OS. If the new lens is as good (with the additional benefit of OS), then it is simply a steal.
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