I was getting

ads_connect: Operations error

when trying to join the domain, it seems somehow related to the version of samba, 3.0.23.

You have to make sure your /etc/hosts file is correct, and that the domain (the FQDN) of your client machine is the same as of the domain of the ad machine.

Also, the client workgroup has to be the same as of the AD workgroup.

Finally, if instead of "net ads join" you use "net join" it will try ads then rpc join, and the last one was the one who worked for me.

Hope it helps.