RDL has an interesting history in terms of playability. It probably hasn't fluctuated as much as Emboar BW has, but their fates have often been closely tied. Breaking down the overall stats, this card doesn't seem to initially be that great. 140 HP is okay, but terrible for a legend. 150 for four seems terribly inefficient when pokemon like Magnezone can do it for three, and worse still, you need to discard at least two and possibly three energy after using the attack. Psychic weakness isn't a big deal now, but can be costly when Mewtwo comes out, and the colorless weakness makes RDL very soft to Tornadus. To top it all off, this is a legend card, so it is painfully difficult to get out. All this comes to beg the question: why would you use such a painstakingly difficult card with so many drawbacks in our metagame?
Space Virus, of course.
Space Virus is interesting as it basically forces the EX effect onto any pokemon RDL knocks out. In other words, RDL takes two prizes when any other pokemon would take one. Against other legendary pokemon or EX pokemon, RDL takes three prizes, and due to the high 150 damage output RDL has to offer, it will almost certainly score that knockout. RDL was very popular in Magneboar's hayday and is considered an important 1-1 card in that deck even today, as Space Virus allows Magneboar to more quickly regain lost ground in the late game. When considering Magneboar's access to Twins, N, and Magnetic Draw, the deck can stay behind by a prize the whole time, then drop a sudden Ozone Buster to win. As long as Magneboar has some kind of board presence, this is a reliable strategy. Some people are heralding it as an amazing counter to Mewtwo EX as well, as it is a card that can OHKO Mewtwo for three prizes. Even though RDL itself gives up two prizes, one smart Ozone Buster means you are halfway to winning the game. If you are able to get another RDL out, you probably have the win regardless of whatever else happens in the game short of all Emboars being KOed. In most situations though, RDL's purpose in life is to be a closer in Emboar variant decks, and this is a job it does very well.
That said, RDL's best friend is Emboar. It allows RDL's attack to become a one drop, making it much safer from Pokemon Catcher. It also allows RDL to use its attack repeatedly as long as Emboar can get three fire energy in hand, or a lightning and two fire. It also pairs well with Magnezone insomuch that Magnetic Draw makes it much easier to get the mighty legend out. RDL also combos well with Twins, as it is a perfect way to get out the legend if you are behind by a prize in a close race, say, 5-4. Shaymin could be used to a lesser degree if a lot of energy is already on the field, drop RDL, drop Shaymin, move the energy, and steal the game. Some might consider this strategy to be overteching, however. Typlosion Prime can also use RDL, but most likely scenario is a two drop for Ozone Buster, giving the opponent a turn to take a shot at RDL. RDL's low hitpoints and damage from Afterburner do not help this, as Zekrom/Reshiram only need one pluspower for the KO, none if RDL was AB'ed on twice, and Tornadus can OHKO it regardless. A Typhlosion player would need three ABs to use Ozone Buster immediately, making RDL very vulnerable for the return KO with only 110 HP. It's certainly a possible option for Typhlosion decks, but not the strongest.
I'll give RDL a 7/10. Part of the lower rating comes from the fall of grace Emboar has suffered, but Next Destinies could really benefit RDL for the better. People few Mewtwo, and taking three prizes on it is a tempting prospect. I would be more sold on the idea of RDL wasn't weak to Mewtwo, but it may still have a use. Smart players can save it for mid game and take three prizes for the win, something that really should not be sneezed at, and with Reshiram EX hitting the field, Emboar might become more playable too. I think the future for RDL is going to be very interesting. I expect it to see play, but how much is something I cannot say at this time.