I must say that this is the only FA EX card that I actually like - Darkrai is possibly the only EX whose texture actually fits that of an EX card (eh, I guess Entei is decent). That and I <3 Darkrai anyway.
Anyway onto the card itself. This Pokemon has become possibly the most hyped card coming out of Dark Explorers. The two main decks that people seem to want to run Darkrai in are Zoroark and a more Darkrai central deck that probably runs Tornadus (EX) to deal with Terrakion.
In Zoroark Darkrai EX is not expected to use its attack very often. Rather, the strategy of the deck is to quickly fill your bench with Dark type Pokemon and attack with Zoroark for at least 120 damage, more with Darkness Claw Special Darkness energy. Zoroark BW is also an important attacker since with a Darkness Claw + Special Dark/PlusPower it can now OHKO Zekrom EX, giving you a large prize lead. Darkrai is really only in this deck because of a rather depressing lack of good Dark type Pokemon in the format. Zoroark BW and DEX are both good and Absol and Weavile are included simply because they do something, mostly the same reason as Darkrai. There really aren't any other Dark Pokemon that do anything handy, and at least Darkrai helps the Durant matchup since Zoroark BW is useless against them and Zoroark DE needs several benched Pokemon, which could be catcher bait if not for Darkrai. That does make it worth including in the deck.
As its own deck, Darkrai is far more potent. 90 and 110 damage are the biggest magic numbers in this format imo, and Darkrai hits them easily-90 just by attacking, 110 with a Darkness Claw or two in any combination of Special Darkness Energy and PlusPower. 90 lets Darkrai OHKO Eelektrik,Zekrom BW after a Bolt Strike, and Aerodactyl while 2HKOing any EX. 110 lets Darkrai OHKO Tornadus, Thundurus, Virizion, and 2HKO any EX with an Eviolite. This is on top of the 30 damage a turn to the bench, so Darkrai can OHKO babies and 30 hp Tynamo, 2HKO Celebi Prime and other assorted basics, and 3HKO Eelektrik and Aerodactyl. Terrakion is probably Darkrai EX's worst matchup since if Darkrai doesn't have an Eviolite it gets OHKOed and even with an Eviolite it needs a Catcher and Special Dark/2 PlusPowers to keep up on the prize trade. Due to this, Darkrai.dek is generally run with things that can beat Terrakion such as Tornadus (EX).
Overall I'm afriad I don't like Darkrai EX's standard artwork as much as other EXes, although it is still nice. Maybe 8/10? The FA form on the other hand is really growing on me, 9-9.5/10.
As a card, Darkrai is much better suited as its own deck with a couple of techs to deal with matchups like Terrakion. I'm giving it an 8.5-9/10 because it will create a top tier deck imo but it will not be BCIF.