As for "most famous and most feared", that's just Orcus being geeked about his and Bill Webb's baby, and a few fans being fans. It is
probably the most infamous d20 dungeon, although I am sure more people are familiar with Return to the Temple of Elemental Evil or the Banewarrens. Larger audience, more fame, that simple. It is not the most deadly dungeon either. Just staying in Necroland, Demons&Devils would give it a run for the money. It
may be the most feared d20 dungeon. But still not as feared as Tomb of Horrors. No way.
It is basically a good, big and deadly dungeon you can spend a few months adventuring in. My players liked Tomb of Abysthor better, but there is something to be said about fearsome - they didn't fear ToA but they feared Rappan Athuk, and they didn't even see the
really bad stuff. So RA:
- is a big dungeon,
- deadly,
- doesn't give a damn about dungeon ecology (there is a bit more than in Tegel Manor, though

),
- is focused on exploration, combat and puzzle solving, with little NPC interaction except "Roll for initiative!"
If you like that sort of thing, RA is worth the money. I am personally happy with my three booklets and won't buy the collector's edition, but that's not a discouragement. That's because I already have most of the material.
If you don't, the answer is the obvious "so don't buy it". Truth to tell, I am fed up with cheerleaders who try to pimp their favourite products to everybody under the sun. That goes for you, Nightfall, especially. It is
blatantly clear that Shemeska would be disappointed by the module if she bought it, and yet you go on trying to sell it to her. Why? You aren't doing her, and yourself, or Necromancer, a service.