Several people on here have made the decision to implant their children, including GrendelQ and Cloggy. You might want to search for their posts (use "search" function and enter name). Both of them are pleased with the results, their children are doing very well, and would do it again.
Also baseballboy was implanted as a young child, and is happy it was done. I believe there are a few others.
There are also a couple people (at least) who were implanted and have had problems, including severe headaches, the devices not functioning properly, etc.
There is a lot of information out there about it, and personal histories, both pro and con. Many of the more vociferous people arguing against it have chosen not to be implanted or not to implant their children, so the arguments are often more from a philosophical point of view than from actual negative experience with implantation.
If there are any on-line support groups for Greek parents of deaf children, that would be a wonderful source for you, as those people would be much more likely to have local experience with Greek or European doctors. That specific experience probably is not much represented here on AD. I think there is one other person from Greece who posts here once in a while, if memory serves.