However, it's an interesting topic.
If I think about where I've lived, then in London it's been South Kensington, Ravenscourt Park, Shepherd's Bush & Earlsfield. Arriving in London it's been Euston, Paddington and Waterloo. In almost none of those cases can I see that I'd transfer from the route I used then to Crossrail if it had existed then.
Perhaps the only exception for me would be Euston-Heathrow, which I'd consider transferring to Crossrail via Northern Line/Tottenham Court Road, but (as stated at the start of this thread) this might be a long walk, in which case I'd stick to my current preference of Northern-Piccadilly change at Leicester Square.
My conclusion is that it doesn't offer me very much and wouldn't have done so in the past either - arriving at Paddington for a Crossrail-served destination might be an exception. However Euston-Paddington I'll continue to use H&C platform at Paddington and Euston Square unless it's really raining hard and i don't have an umbrella with me.
My sister now lives in Greece but owns a house in Tilehurst, so it's probably good for her and the value of her house. When she lived in Twyford, though, I tended to go to Reading and in which case I'd continue to use Paddington main line.
I'm sure it'll get plenty of use, but I don't think by me, I'll probably use it out of curiosity once and that'll be that.
EDIT From what I can see, it looks like a reasonably short walk from the south end of the Northern Line platforms to/from Crossrail at Tottenham Court Road. So, good, I'll give it a try next time I want to get to Heathrow, say.