HP don't provide any power management features with their T5735 units (i don't know about other models) which i found most annoying. I've tried this and it works pretty well, HP say they don't support it but it's so simple i can't imagine there being many issues.
It basically turns off the client if it's not in use but you can set core hours that you don't want it to work so you can say don't turn off between 9 and 5. I've got it set to not turn off for 1 hour after it is turned on and then it turns off if it hasn't been used for more than 15 minutes and it seems to be working pretty well so far.

Autopoweroff: User Guide