Personally, I would never accept a discount, a free extension/free time or chargeable 'extras' at no cost.
But what I would accept is a simple apology and a seriously positive attitude during the subsequent meeting.
Why not accept such a thing?
I had one of those experiences: after much comms trying to work out a time that would work for me and a girl-girl combo who were advertising, come the day there's no answer to their phone, even when in the street where they live. They appear to be apologetic, and blame a faulty phone. However the apology only appears when I mention negative reviews, and is accompanied by a threat of legal action (!) should I do so.
They offer a rebook, and I say that in view of the fact that I've already driven an hour each way to see them, they should offer a discount or extra time. They say they have a strict no-discount policy, but that they'll be especially nice if I rebook.
I didn't book again - to me, a discount or extra time is a proof of sincerity: without _something like that, what's there to suggest that the exact same thing won't happen again?
In the end I didn't make a separate neg review (should have though for the review count!
), just posted the story as a comment to one of their existing reviews.