Disclaimer: I build this plugin!
But I build it with a purpose in mind; getting my small webshop into Google Shopping next to the big boys. Although the plugin currently only supports one Google Shopping Category for the whole feed, that’s enough for my 1-product-type shop.
It took me quite a while to get the feed just right. Google only accepts very specific feeds. But now it’s working like a charm, and my products shop up for any related search. Very handy and boosting my traffic and sales a lot.