Based on the Howey test it seems like it could be argued that a REP token hits all these indicators.
It is an investment of money?
There is an expectation of profits from the investment?
Yes the REP token is one of the few tokens which should generate income.
The investment of money is in a common enterprise?
Yes, I think its fair to call the Augur network or even any individual market within it a "common enterprise".
Does the potential profit come from the efforts of a promoter or third party?
Potentially yes as the REP holders cannot earn a profit without the efforts of the market creators.
I'd love to hear other interpretations.