A very easy to use extenstion to generate an XML file for the Google Product Review Feeds (https://developers.google.com/product-review-feeds/). It reads all your reviews from all your products and generates a product feed as required for google.
The extension includes the possibility to include the IDs which are highly recommended. You also get the possibility to choose how to split the IDs if you are using multiple values for one or more products.
Here a small screenshot of the extension: