I just listened to Det Som Engang Var for the first time tonight and I have to say it was worth forking over the 20 dollars. The album just got better with each song. "Lost Wisdom" has become one my favorite Burzum songs with just one listen. I can see how Hvis... was natural progression from the work on this album, which sound like more standard black metal mixed with the atmospheric stuff found on later albums.
Any fans of USBM must listen to this album to fully appreciate where it all came from.