Definition of the word Experiment:
A scientific procedure undertaken to make a discovery, test a hypothesis, or demonstrate a known fact.
If Bitcoin was/is an experiment, I don't think it's trying to make a discovery, it certainly isn't demonstrating a known fact.
It might be more closer to testing an hypothesis like "A digital decentralized form of money is viable". But what exactly is the hypothesis? Where does the test end? Who declares the end of the experiment? The bitcoin foundation? The governments? Forum folk?
As we're nearing the end of 2014 and getting closer to the fifth birthday of Bitcoin, is it still an experiment?
I know that this is a very broad and open ended question. And yet, the argument is popularly used by prominent people from the community. Like Gavin Andresen. Everytime he faces a tough question, this is his response. If we don't know where the experiment began and where it ended or where it will end, then maybe we should not use this as a wildcard answer.