It's worth what we as a community decide it is worth. If everyone decided to stop mining tomorrow, it would be worth nothing. If we all decided it's useful as medium of exchange and store of value, then it is worth more. How much more depends on what how many people use it and for what. It's an exciting experiment.
It would not be worth nothing if the mining stopped because what has been mined would still exist. And there are only 11 million bitcoins in existence. That is very little compared to silver bullion for example. Perhaps that is another way to put a value on it - compare it to silver and gold?
Maybe Max is right. Could this really be headed to 100,000 or a million per bitcoin?
True, but if the miners quit, the blockchain could not be built on and transactions would stop. Perhaps, people could take that blockchain and decide it's the "true blockchain" build a protocol that doesn't have miners, but "transactors," and continue on with a new protocol, but would that still be Bitcoin? It's almost a Ship of Theseus paradox.
I think deciding to "invest" in bitcoin as a form of speculation is a bad idea for the community. We have infrastructure, goods, and services to further the economy. A little bit of speculation isn't harmful, but we need to keep our thinking grounded in the present, for the future's sake.