When is an FPGA mature enough?
Ask your FPGA vendor's distributor or FAE.
Do you need the latest? If an older part is big and fast enough, don't use the newer ones.
If you need to sign an NDA or get beta software, the part isn't mature enough.
Is the new chip cheaper or lower power? If not, consider sticking with the old part.
If the full line of chips (say Virtex 7) is out, it is probably mature enough. If they only have certain sizes an packages, it is still relatively new.