Read it and enjoyed it, but not enough to read it again. I liked the start more than the end, probably because it was still an amusing twist on Harry Potter. The further the story progressed, the more it became the author's own wildly divergent creation with its own rules and quirks. While that's not an objectively bad thing, it wasn't a world I particularly liked. I was charmed by "rational" Harry interacting with and trying to make sense of a brand new magical world, and I was very much able to suspend disbelief and accept this ridiculously precocious 10 year old protagonist. That slowly eroded as other characters in his year proved just as precocious; most of the kids didn't actually seem like kids. I still liked it well enough, but the writing at the end felt a tad too clinical for my tastes.
IIRC, the best thing I got out of it was one of the Author's Notes that linked to
Worm, one of my absolute favourite works of fiction.
If you liked HPMOR, you might also enjoy
Harry Potter and the Natural 20. The basic premise -- a D&D Wizard gets dropped into Harry Potter. Word of warning, the author is on an extended hiatus (I think), with the last update was about 15 months ago, relatively early into the author's version of Prisoner of Azkaban. If that bothers you, you could stop after the end of their Chamber of Secrets parallel and have enough closure.