It's simply too expensive -- there are ways to make it useful, but for eight mana you can get much more powerful things that don't require that kind of finessing.

One interesting note, though: You can use its ability even if you fail to pay the upkeep by activating it in response to the trigger. This is less useful than it seems, because the sacrifice is not optional, but it's good to know. It's also something to be very wary about if you plan on giving him to your opponent -- remember, they can still tap him during their upkeep in response to the trigger, and use his ability to take your creatures (or whatever you took last turn.)