I don't think tales of past upgrades is particularly relevant, seeing as the code base is now MASSIVELY different to what it was a few years ago.

On saying that, I found the transition from 6 > 8 > 8.5 absolutely painless.

This cannot be said of the transition from 8.5 > X1, but this was largely due to X1 being a brand new version and not just a simple upgrade, which meant the original X1 was a bit of a bug fest.

X1a didn't help, nor did X1b - it was only when X1c came out that I regained the stability I was used to.

But X2 is a continuing evolution of X1, so I fully expect the transition to work as expected.

I have 3 machines on which to try out X2, so I'll first install it on my secondary DAW before committing it to my main rig, "just in case"