...the general consensus among the people at the prerelease was that this set rocks during sealed deck. It also has tremendous pack-opening appeal, as a third of the 45 rares in the set are either a wind, an avatar or a legendary spellshaper. However, there is also a general consensus that this set isn't really that strong in Constructed deck events due to the extreme expense of the cards (in terms of mana and drawbacks, of course). Plus, many cards in this set require land sacs, or can be countered if an opponent pays so much mana, or require all your lands to be tapped, so I think that, at least until Urza block rotates out, Prophecy will not see much play time except in certain decks that can either churn out a lot of mana or benefit from having little or no available lands, or destroy all opposing lands.
Still, I wouldn't pass up buying a box, either, at a reasonable price. This set still has the potential to be incredibly strong, although I'm not certain if the strength is in the rares or in the commons.