I don't use ecommerce, but I've really been impressed with the approach taken with calendar:

Yes, everyone agreed it needed to be rewritten, but Dirk has been implementing the changes in small, easily obtainable chunks. I'd hate to see someone undertake such a large project and not completely finish it, or lose functionality. Then we'd be stuck with two ecommerce packages we don't like.

Instead, why don't we gradually fix it?