For me, Phyrexian Dreadnought (Mirage) is just as much a classic Magic card as Ball Lightning, Howling Mine, or Lightning Bolt. It's one that I'll always remember. I'm not sure if is had (has) any competitive appeal, but I know it has a huge casual appeal. Oddly, I came to really like this card not from playing it, but having it played against me.
I generally stuck to my Burn, Discard, or Mill decks. When I would see cards like Phyrexian Dreadnought, I would be blinded by the thought that the cost was just too much to make it worthwhile. Not the casting cost naturally, but sacrificing creatures to a combined power total of 12. You can't even cheat it in, because it triggers when entering the Battlefield, not cast. Also, I rarely played creature decks, so I'd never have the fodder to begin with.
As it turns out, a lot of times this wasn't played in decks full of creatures. Rather a mix of creatures and creature control. If I remember correctly, friends would play this in Chaos matches along with Reins of Power (Stronghold). At least that's the most prominent card that comes to mind when I think of Phyrexian Dreadnought. So a player steals all the creatures from another player, attacks with those creatures, and then essentially sacrifices them to the Dreadnought. It was pretty epic when it happened!
The card has always been relatively cheap, and my copies (as seen above) are in fairly rough condition. They may be seeing a price hike, but as for why, I've no idea. Either way, just from researching the price I discovered there was a foil Judge promo printing of Phyrexian Dreadnought. That's awesome! So... I'm hoping that flux is a mistake and it settles to a reasonable price. At least until I purchase a playset...
Have you ever played with Phyrexian Dreadnought? If so, what decks did you play it in? What methods did you use to mitigate the high "enter the battlefield" conditions? Have you ever played with Reins of Power? If so, what kind of decks did you use it it?