Sentry Totem is an Air totem, and thus cannot be used at the same time as another air totem. While it has slightly more hit points than other shaman totems, it is still easily targeted and destroyed. As long as the totem is 'alive', the shaman receives a buff that when right clicked switches back and forth between seeing what the totem sees. The buff provides no actual benefits in combat, and is purely for switching viewpoint.
Since a shaman can already use Far Sight to keep track of things happening far away, the totem's main uses are indoors where Far Sight may not be used, and as a simple "early warning" system hoping that any enemy that finds it will destroy it.
Sentry Totem is popular when taking screenshots, especially boss kill pictures in raid instances, as it allows the 'camera' to be placed anywhere relative to the shaman. This lets the shaman pose along with everyone else while still allowing the best possible camera angle to be used.
The totem is highly useful in Warsong Gulch and when defending towers and bunkers in Alterac Valley, as the totem can be placed at an entrance, and anyone who finds it will generally attack and destroy it, warning the shaman of their presence.