Have loved them since Divine Conspiracy, though for some reason I never bought Requiem for the Indifferent.
I think I prefer Quantum Enigma to Holographic Principle, though I haven't listened to the latter much since it came out. Need to rectify that, immediately.
I'm completely the opposite on Quantum Enigma vs. Holographic Principle. I bought Quantum Enigma when it came out, listened a few times, and just kind of forgot about it. No real reason. It just didn't really grab me. I think a lot of that might have to do with the fact that Retrospect had come out not long before, and when I wanted my Epica fix, Retrospect was still my go-to (it still is, for the most part). I guess the thing for me is, I wouldn't really call myself a hard core fan. They aren't a band I have followed all that closely. But I have come to like their sound. Retrospect is perfect because it's such a great collection of songs and such a great performance. But in terms of individual studio albums, Quantum Enigma did little to make me stand up and take notice of individual songs, so it quickly faded.
I bought Holographic Principle on release and didn't really care for it on first listen. But a big part of the reason I didn't like it turned into a positive over the long run. It sounded different. This is probably the first album of theirs where I have noticed a marked change in sound from them, and it kind of threw me at first. But after revisiting, the individual songs really took on a life of their own, and each song definitely has a unique identity, which I feel had been missing from a lot of their material in recent years. And the OUTSTANDING acoustic bonus disk was a huge plus as well.
I didn't buy Design Your Universe or Requiem. But to my ears, those two albums and Quantum Enigma just sounded kind of samey. Not that there is anything wrong with that. But as someone who is maybe just a bit more than a casual fan, there just wasn't enough there to pull me in. So Quantum Enigma just sat on the shelf. Honestly, I forgot I even had it until your post, which made me go to my CD shelf and pull it out to bring to work and listen to it this morning. I ripped it to the work computer, so I will likely be listening to it a lot more now. And these guys are kind of "trending up" for me. I've been noticing myself wanting to listen to them a bit more, and wanting to reach outside of what I have on the computer here. So I can see myself expanding my Epica collection in the not-too-distant future. (I presently only have The Divine Conspiracy, The Classical Conspiracy, Retrospect, The Quantum Enigma, and The Holographic Principle) But we'll see. These guys are kind of a challenging band for me in that lyrics are generally an important part of the picture for me, and I disagree vehemently with where they are coming from philosophically in a lot of their lyrics. That isn't as hard to reconcile when I am just enjoying them as background music during the workday. But as I get more actively engaged with their songs, it may become an issue. We'll see.