Once is decent, and has its obvious classics, but it also has the atrocious "Wish I Had an Angel." I think the album is okay, and Planet Hell and Ghost Love Score are frequently played, but otherwise, I don't get Once.
I don't really see how it stands up to Oceanborn, Wishmaster, or Century Child, which are their three best and most consistent albums IMO. I think the problems I had with their two more recent albums started to rear their head in Once.
I wish I could appreciate it a little more, since I frequently hear the opinion that it's their best, but it just strikes me as boring when compared to those three prior albums. I hope they drop the Danny Elfman stuff for the next albums, because it's been a serious obstacle to my enjoyment of their music.