So I decided to join the "club" and try Symfony2 finaly.
I have to say that I am not familiar with most of this framework and it's power but from what I saw and read till now it seems to be promising.
I am not going to describe much about Symfony2 as I think there is enough documentation out there regarding to that.
More than that, Symfony2 has a great documentation and examples which make everything straight forward.
Class "Acme\StoreBundle\Entity\Product" sub class of "" is not a valid entity or mapped super class.
The error occurred after trying the ORM Repository example.
I Googled the issue for sometime and found two available websites that were suggesting solutions for the above issue:
One of the suggested solution was to make sure that eAccelerator is not running as it might cause problems.
I checked my PHP modules (php -m | grep -i "eaccelerator") and I found out that I was using it and removed it (from php.ini or php.d/eaccelerator.ini).
After doing that I restarted Apache and everything worked as expected
Basically this issue is related to Doctrine but I was working with Symfony so I preferred to post about both.
Hope I helped,