It depends on what you want to do with it. Obviously, if you have a full copy of Mathematica then it will be of no use to you personally.
However if you want to share notebooks with others then it is invaluable. For example, an instructor could create interactive lecture notes using Mathematica and students could view and interact with them (but not edit them) using the player.
The ability to actually perform calculations in player is limited to evaluating Manipulate functions (and even then only with certain controls) but with a bit of imagination you can do a lot with this.
The player can run everything in the Wolfram Demonstrations project for example.
Feel free to get in touch if you’d like to discuss this further – you have my email address.