I don't think Anathem had a good ending per-se; rather the whole novel sets up a gimmick revealed at the end that explains away the lack of a good ending.
I think a well set up gimmick is still a gimmick. And I think probably for the first two thirds of the book at least, the reader will have no clue what's coming even though it's being set up.
Anyway I do like the book, but the ending wraps up very abruptly thanks to the employment of this gimmick.
Going by the comments on this thread, it's hard to believe everyone's talking about the same book... (which I haven't read, and I'm now torn as to whether I should!)
It's doesn't go "off the rails" at all, it completely and satisfactorily explains everything. It goes fast but it's good. I highly recommend it. You won't want to miss the worldbuilding, it's absolutely incredible.
Yeah it's wild! Second-guessed myself a tiny bit after seeing all the conflicting views.
Minor spoilers:
I loved the world-building and mystery in the first half, the main character lives in a post-apoc monastary, tons of great details and tidbits. It felt vibrant, deep, reminded me of Redwall and the 5th Harry Potter, with the sexual repression and small-scale rebellion against authority.
He leaves the walls in the second half and stuff happens, I forgot most of it. The journey part felt like an afterthought. Something something aliens, really didn't feel earned or connected.
It's still my favorite Stephenson novel though.