Variant inventory is crucial. I've been using a solution that worked well for 2 years and when we had to 'upgrade' to a new version for a new store, inventory is no longer updated.
Selling things online that sold in the store, but didn't update inventory because the $1000 software for inventory isn't working is pissing me off.
I would use XML, bu they don't have size/color in inventory sheets, only total numbers.