i used opera for a long time, and i felt that the turbo is worth activating.
turbo uses the opera servers to make a cache of the site we are surfing and provide the content to us after that, in this process the original images on the page are compressed and served for a faster web experience.
when it comes to the data saving , it is common sense that if data is served compressed then it will be less than actual.
but now i am using firefox 25, as firefox serves best if you use to surf the sites frequently, as it save the cache etc.
surf a site first time it will take some time, if you open again and again, once it will be instant loading of that site.
i recommend use of mozilla firefox.