I've used the advance upload function a few times flying to ATH (from various outstations). Sometimes you get a confirmation, sometimes not. My impression is that the KLM service is quite a bit faster (and more consistent) processing these.

Ex ATH they have never offered this to me though (Which sucks because I'm typically only able to arrive at the airport shortly before departure). In fact, even after receiving the BP at the check-in desk at ATH it will not feature the Ready2Fly logo, causing all alarm bells going off at CDG when flying onwards. Last monday I got multiple SMS messages upon arrival at CDG to go to the gate early and have my booking fixed. Instead I went to the lounge...but they were unable to do it.