installer pretty much has nothing in it apart from the region patch which at the moment you will need in cyprus unless you enjoy saving numbers twice once with +357 and once without for each contact... i got my phone from england the app store works just fine i download games and apps and i use cydia and installer games and apps including the much needed region patch... of course i jailbreak my phone!!! i love my jailbroken phone i made my own theme i download apps like vlc player and yes most of my apps are from app store but region patch from installer which i badly need , i can ssh into my phone and change all my ringtones on my phone to ones i want and i can edit files so i can download the pay for app store games free
sure it voids the woranty but i know what i am doing there is no way i break it espesially with the progam i use to do it, its very safe but once you do jailbreak if you need help you have to pretend to apple you did not jailbreak it or ask in other places for help. even if you just unlock it then that still voids the worranty
so the fact that you will have it from greece means either its unlocked from apple like kypraia has or you will use some sim solution to have ti unlocked...
If you dont care about those things then there is no point jailbreaking.