U can use it as a digital tuner, and a half hour time slip. And if it has lifetime service you can use all the features. If it has evaluation you can use most of the features.
From TiVo Central: Go to Messages & Settings > Account & System Information > System Information.
to see if it has lifetime service. If it says 'closed' then you can only use it as a tuner or watch the shows that were recorded on it before.
For some older TiVos you were still able to record manually if the account hadn't been closed before the TiVo was disconnected from the phone or internet connection. I don't know if that was ever true of the HD TiVo.
I do know that the latest software does make it so if the TiVo isn't connected to the phone line or internet it will stop recording. Even if it has lifetime or evaluation service.