Item Rating is definitively the most powerful system to display product reviews and ratings in Joomla, K2 and multiple other Joomla extensions. The extension has the voting flexibility you've ever had before. You can create unlimited rating group with criteria inside. Each criteria can be a different voting tool like percent bar, star, icons or thumb up/down. If you're a webdesigner, you'll also like the custom design tool in rating group with element positions definition. Item rating got also a powerful voting restriction: you can allow vote only from backend, from frontend, from both, allow only certain Joomla user group.