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Briefly, yes the lens is a little heavy (around 3lbs), it does seem to focus fairly quickly, and it is certainly hand-holdable for me. My main interests with my canon lens is shooting bugs and butterflies around my backyard.
Personally, I'd rather have the much lighter Tamron 180/3.5 for this. While I admire Sigma for their courage to produce it I think it's a step too long. My dream long macro lens is an even lighter and smaller 200/4 with OS and HSM.