Depends totally on your situation I think.
When I got my first full time proper job to live off I lived in an awful flat in Manchester (reasonably cheap housing) and as a couple we made it on my £17k a year. It was a good salary for someone who was just a student.
A year later I was on £24k a year, which at the time I considered a good salary. But we were actually worse off, as we got a cheap-to-run car (that's never going to be cheap enough) and we moved to Bristol for the work, which was much more expensive.
Few years later I was on £31k, and my partner was working mostly full time. I considered that a great salary, we could afford a house, better car. We were making over-payments on the mortgage.
Now I'm on £38k. I consider that a very good salary. But we had a kid. Kid+House+Car-One Income. Financially, we're much worse off. But considering we can cope, these days a single salary is still pretty good.
Would I still think £17k is good though? No. Nor £24k. I've got so many more commitments, we'd struggle with less than £30 I think, given the cost of gaining an income again (childcare costs).
Very much depends on where you are in life and your situation. I've gone from low salary to high salary, and I've never felt that money has been flowing easy. All my increases in salary have been absorbed by a slowly increasing lifestyle cost that you really don't notice. I'm not surprised people who earn 50k don't feel well off.