Or to phrase it another way, how does sigma's 18-50 compare to the rebel kit's 18-55?

A better place to ask this question is probably the Canon SLR Lens forum over at dpreview:

http://forums.dpreview.com/forums/forum.asp?forum=1029

Taking a quick look it appears that the general concensus is that the two are optically comparable, but the Sigma has a noisy focus motor.
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