I've had nothing but problems since upgrading to 5.1.2. On two different machines.
After a boot, it will work fine for a while, but it'll start tossing bogus errors with increasing frequency:
SSL certificate errors
invalid URL
permission errors, unable to access resource
inability to select text in a web page
and other odd issues
Happens with multiple sites. Set Facebook to use https and surf facebook for an hour. You'll have problems.
Using another account works around the problem for a while, clearing cache or relaunching Safari does not. Rebooting clears the issue for a while.