I bought Delta Green as soon as it came out, based on the combination of a positive review in the much-missed Arcane, my then-obsession with The X-Files -- I was young and foolish -- and my then-and-never-diminished love of Call of Cthulhu. I read it. I ran a few sessions. I didn't like it much.
Part of the problem was that there are too many conspiracies in the setting; everyone's involved in some multi-layered cabal, to an almost comedic extent; I can imagine that there's some potential in a light-hearted Delta Green game in the style of Over the Edge or Paranoia. Another issue was that some aspects of the setting were just naff, like The Fate, all trench coats and shades, like a poor man's World of Darkness.
The biggest problem with it is one that didn't become apparent until I encountered the setting as a player, as I did this past Friday. There is a lot going on in the Delta Green universe, and there are a lot of non-player characters involved in the various plots; the thing is, the NPCs remain elusive and untouchable, insulated from player contact by layers of narrative immunity as they go about having fun adventures while the players stand and watch. There's something rather familiar about all of that.
Yes, it seems that Delta Green is Call of Cthulhu's equivalent to the Forgotten Realms. No wonder I don't like it.