i'm a sigma 35mm early adopter, and i'm very pleased with it, but if i was you, i doubt i'd swap. if you go the ebay route, then you have ebay/paypal fees, plus and maybe it's just me but i always get hosed on shipping charges, and all that will eat into your rewards. If you have a cheaper route, and are unhappy with your canon, of course, but if your happy, i doubt i'd do it. I was minutes away from going with the canon 35 when i decided against it, but if i had, i bet i would be just about as pleased with it.
- just like to add that i think sigma's rapid lens replacement system is a detriment. Canon is more my speed. Hopefully sigma will slow things down.