My parents divorced when I was an adult, and my mum has a long term partner that I get on very well with. They are much better suited than my mum and dad were. My dad also has a new relationhip, but it is less developed.
It's probably different whan it begins as a relationship between adults. There's less pressure and less expectation I think.
To my children my mum's partner is certainly a grandparent although we all use his first name.
I know his children, but don't regard them really as family, which is odd, because I would certainly count him as family.
My mum's integration with his family was a bit more troublesome, but is resolved now.
It's a good situation really