Indeed it's up to Google whether to display the results or not. Moreover, it can display it for 50% of your products and do not display for the others.
Actually, there are lot of confirmations that the module works and rich snippets are displayed (including posts in this thread from BritSteve, recent post by DavyMac and others).
As I have already mentioned in this thread there are might be several reasons why Google doesn't display snippets. Please refer to the "Why is Google not showing rich snippets for my pages?" section of the following article:
http://wiki.goodrelations-vocabulary.org/FFAQ#Why_is_Google_not_showing_rich_snippets_for_m y_pages.3F
Hope it helps.