Darkrai MD is a very debatable card. Its easily the fastest, and most sleep inducing of the darkrai's, but the effects on some of the attacks can come out a bit after they are useful.
The pokepower on this guy is a beast in a dedicated Darkrai deck, especially if your frontrunner is Darkrai GE (#4). Slapping someone for 80 on turn 3 may not be terribally gamebreaking anymore, but its still very effective for what easily searchable basics can do. Note that it loses a lot of its luster in a doubles match. Also be warned, this is a power, and boh Gardevior and Aerodaytl will have fun with it.
The first attack, while not looking like anything spectacular, is actually a lot more savage than the slow effect would make it seem. It would be infinately better if it cost (C), but (D) is doable, especially if you draw a special dark first turn with it. Note, this play can knock out ascention shuppit on the very first turn of the game. The thought of sleep coming can also force the opponent into unwanted switches very often, allowing this card to have more of a role as energy disruption than offensive tank. Imagine Magmortor or Blissy trying to retreat, dropping a scrambles worth energy while they run.
Speaking of tanks, thats exactly what the second attack does. If, by some means you have the opponent asleep, Darkrai can almost completely heal itself. Big note on this attack, its not like sleep hole on Darkrai GE (#4), its either effect or not, no split road. A good way to abuse this is with Sidney's Stadium and Wiggly Tuff, requiring that you not need switch Darkrai every turn. Once this guy starts healing, he's pretty much locked in, unless you've either got a lucky opponent, or the other pokemon faints.
Weakness, resistance (note the resistance btw, this thing's a nightmare for Gardevior), retreat cost, and hp are nothing special for a legendary. Gallade can deliver to OHKO if no DRE it attached, without even flipping a prize. Otherwise, Darkai fears not a single card in the Gardevior line, as ralts can't really damage it (especially with sidney's), Kirlia and Gardevior are better directing their attention elsewhere, and even Bring Down on lvl X may fail, as Darkrai's hp is higher than a good number of techs, and perfectly even with Claydol (note, 10 damage on bench = happy day). Gardevior's teleport power however, may end up messing with your sleep stratagies, so try and sleeplock her in the front row.
The big factoid that makes this perticular darkrai amazing is just how well it works with Darkrai lvl X. A Darkrai X on the bench means this Darkrai should be able to come out swinging a minimum of 20, or even 70 if its been preped for its second attack (which puts Gallade well within distance for a Bringdown against most dark decks). If Darkrai lvl X locked in his attack, this gives Darkrai MD time to completely heal itself, and if you decide to run moonlight stadium instead of sidney's stadium (especially out of fear of the mirrior match), this may happen more often than you think.
As a darkrai, this one serves best as either your starter or your finisher, as its the only darkrai with an effect that benifits even after the opposing pokemon is knocked out, as well as being the only one that can force early damage. It is easily the best opener a pure dark deck ever had, and perhaps the single greatest force drafting may ever see.
6/10 Normal
8/10 Pure Dark
10/10 Draft