Memory savers are useful when changing car batteries, but not essential if you have the radio codes and alarm key fobs etc. But can save valuable time re-programming radios and clocks etc.
Factory fitted immobilizers will work fine after a battery disconnect, but beware that most remote alarms will arm, after a battery disconnect and you will need the remote fob to disarm the alarm.
The best memory savers will have a LED that will illuminate when you first connect the saver to the car, this will confirm that you are connected, they will also have a meter/LED that tells you the memory savers battery (or connection to slave battery) is good.
They connect either directly to the battery connections or via the cigarette lighter if it is live without the ignition on.
Be sure not to let the positive lead touch the body or open the doors, when disconnecting the battery, as the load can cause the memory savers overload trip to disconnect the memory saver.