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Do you think they will actually deliver a working product, or at least a beta version, on the deadline, or is the tune going to change at some point?

For example, the Golem team just keeps pushing their deadlines for the mainnet Brass release.

Yes/No.. Generally I expect delays mostly to be in the Ethereum backbone and Plasma development... that's almost a given IMO.
However, I must say that because OMG has this tiered approach, that means they are developing parts to be working 'in the meantime' while they wait for the Plasma parts to be done.

I do expect OMG to implement staking soon (they said so) and I do expect a working wallet soon too. My personal idea is to start looking for sell options in Q1 2018

I'll be very impressed if they get the wallet and staking done by Q1 of 2018.