It's a good start... I found it easy to set up, publishing our products in Google Merchant Center quickly.
So why not 5 stars?
Google Merchant Center complains about various issues and I can see from other reviews, it's not just me. Specifically, I had an error reported in Merchant Center about being unable to automatically update the products and believe this was due to the product notifications about the condition and price missing in the schema.org.
Shopify support tried but wasn't able to really help. They pointed me to Google, suggested a few things, etc. but to no avail. I believe I was just able to over come these issues (waiting for Merchant Center to refresh / re-sync) with 2 resources:
1) Google Structured Data Testing Tool (https://search.google.com/structured-data/testing-tool#) and
2) Alex Czarto's web site (https://business.czarto.com/, specifically https://business.czarto.com/2016/12/13/adding-schema-org-microdata-to-your-shopify-product-pages-to-enable-google-merchant-center-automatic-updates-with-product-variant-support/)
Overall, I think it's a great app. It just needs better support and to comply with Google Merchant Center's schema needs fully. Spent a lot of time trying to sort this and copying various bits of remarketing code (which caused me some other problems) from other sites to get it fully working - I hope...