Google has not correctly implemented DNSSEC. If you send them a normal DNS query and the response is not validly signed, they just pass the answer back to you without any indication that it's invalid. They only tell you that the answer failed to validate if you set the DO ("dnssec okay") or AD ("authentic data") bits in your query, which almost no DNS clients currently do.
If the answer is invalid, a validating name server is supposed to respond with SERVFAIL, so that even if the client doesn't know anything about DNS security, it will still be protected against spoofing. Google is claiming to provide protection against spoofing, and then they aren't providing *any protection at all*.
If you want DNSSEC protection, you're still going to have to run a validating name server yourself: either BIND 9 or Unbound. (Disclosure: I'm a BIND 9 author.) It is, nowadays, extremely easy to configure a validating name server using BIND 9; in any version since 9.8.0, a one-line named.conf will do it:
options { dnssec-validation auto; };
Run named with that configuration and "nameserver 127.0.0.1" in resolv.conf and you're good to go. Google public DNS is not ready to trust yet.