EJB Timers

Hello All,

Has anybody used EJB timers which are part of EJB2.1? Do these timers slow down or have any performance problems. We have option of using it in real time application and am wondering if they are relaiable and guaranteed to kick off at the specified trigger. I am especially talking about the ongoing timer in Timer interface which will trigger after every twenty minutes.
A related questions, these timer are set only when the EJB is invoked first time. Is there any mechanism ( I couldn't find in EJB2.1) where you specify option in the deployment descriptor to set or not to set the timer after deployment is complete.

Thanks to all.
-Ahmed