I've tried the free gh/s but i don't know if it's a great idea to continue "mining" there
To withdraw the funds from your free-miner you have to invest
that reminds me to a 'piramidal' word

Um, yes.. the free miner is for testing purchases. We would go bankrupt if we allowed people to withdraw without purchasing, we tried that at first and ended up with 2000 signups with 100 views on our bitcointalk thread.
-Sahra