Depends on what you mean by "really safe"; strictly speaking, there is no such thing as "really" safe; there is at best "safe enough". SSL has known weaknesses (which is not the same as vulnerabilities); Bruce Schneier somewhere complains about SSL's too permissive public key infrastructure, but without that permissiveness GC would not be able to use a self-signed certificate.

Of course it also depends on what you do; for a site like GC a self-signed certificate is safe enough, but of course not for something like internet banking.

And check the "Use SSL 3.0" and "Use TLS 1.0" options in your browser's settings (Options|Advanced|Encryption|Protocols in Firefox).

Surf safe;)
NFiH