Ermelindo,
Correct me if I'm wrong but the OpticZoom 5x belongs to a company called EagleEye right? Well, like you, I was interested in getting the OpticZoom 5x. Therefore, I requested for more info from folks @ EagleEye. Here's what I got from our correspondence.
Our OpticZoom 5x Telephoto Lens can be used with the G3 but as with most cameras when you attach any add-on lens there are vignetting issues to consider. With the G3 you are pretty much confined to using the 4x camera optical zoom setting. The combination does produce a slight amount of vignetting even at maximum camera zoom. We do not have any sample images for the G3, but if you take a look at the Sony S70 sample images on the website the extent of the vignetting is very similar.
It's the vignetting that I can't live without. I think, the further you go with the G3, the more vignetting it's got. Well, do update us upon your purchase. (i.e. if you intend to, k?) HTH !