Lucasfilm Games really experimented a lot, these early games are expressions of their creative freedom. A luxury that most game development groups/corporations didn't, and especially nowadays, don't always have.
The manual is incredible, inspiring and atmospheric, I recommend everyone to read it fully and admire the great art. The whole story is unique (never seen this story anywhere else), the idea is sound and mysterious, and everything in the game supports this.
However, I found the gameplay a little cumbersome and boring. It doesn't quite give the player the 'feel of an epic adventure', but more like 'how do I complete this level so I can get to the next, almost identical level with more difficulty'-feel.
It's not about exploring (the caves look the same everywhere), it's about jumping through annoying hoops to advance to next level, then doing that again and again.
The drawn art is great, of course (done by same artist that created the amazing Maniac Mansion on the C64 side), the game is very ambitious for its time, but I find the 'fractal effect' doesn't really work, it looks a bit clunky and chaotic instead of precise and three-dimensional (but so does 'Rescue on Fractalus' (try to spell this right, people! It's not 'Fractus' or 'fractulas' - it's based on FRACTALS, so of course it's FRACTALUS).
I could never really get much joy out of Eidolon or Koronis Rift - the technical side is very impressive, and there's a lot of atmosphere, but the gameplay just seems more 'annoying' than 'fun'.
However, I do enjoy 'Rescue on Fractalus', because its gameplay is tight (although the 'mountains' really just look like chaotic lines instead of proper 3D again).
As a sidenote; I really only learned to not only like, but seriously love 'Rescue on Fractalus' only after playing the Atari version. The C64-version is so bad that although I wanted to like it, I couldn't.
Atari version has everything done correctly, from intro to sound effects to visual eyecandy to playability - and it's fast enough to actually enjoy playing.
I don't know how different the C64-version of Eidolon is - I think I played both of the versions on real machines the other day, but can't remember well enough to really compare. I suppose they're very similar in most respects, except C64-version has a bit more boring soundworld and less bright graphics.
The Eidolon is an OK game, but the gameplay itself is a bit of a simplistic let-down, and the cave always looks the same, so the only variance is the beautiful 'monster art' and the incredible manual - both of which I enjoyed more than the gameplay itself.
This is the price of 'experimenting' - sometimes you get inspited, vision-based, ingenious gems, and other times you fall flat on you face, because the experiment didn't quite work. The Eidolon, for me, lives somewhere between Rescue on Fractalus and Koronis Rift, as I think the latter is even more boring and annoying to play than this.
I might be a bit biased, since I think Maniac Mansion is the best game I have ever experienced, on all possible levels, except maybe a bit of annoyance because you have to keep running back and forth between upstairs and downstairs.
So I do enjoy these early games somewhat, but in my eyes, they never quite reach to the level of the Mansion. However, Rescue on Fractalus is the perfect "pre-Maniac Mansion" game from this brilliant Lucasfilm Games group, The Eidolon I can 'give or take' in short servings.