I'd personally like a bit more HP as D/P is crammed with powerful, reasonably easily charged attacks, but I still wouldn't like facing this card. Ultra Powder can be really nasty although its a bit unreliable; I prefer choosing the status condition to inflict myself rather than letting the number of coins you've flipped decide what status condition you'll inflict with heads, but with a bit of luck, Ultra Powder will really pay off. Fire Counterattack is mediocre unless your opponent is running a fire deck, but if he/she does, then it'll become a great way to counter the typical fire weakness many grass types (including Meganium) posses.
The resistance is good because it allows Meganium to survive 2 fully charged Water Guns from Floatzel, gives protection to Empoleon's Aqua Jet (which would otherwise KO it in 2 blows) and makes it practically invincible against some of the weaker Water types your opponent might be hiding behind while they charge up a Pokémon on their bench. The retreat cost is good for a Stage 2 and the weakness is tolerable because it'll still survive a Flare Blitz from Infernape as long as it hasn't been damaged.
8/10; with the right support Meganium will stick in play for a long time and annoy your opponent with various status conditions and KO some of the weaker Pokémon with Fire Counterattack. Some of the less durable fire types will have to be careful as well. The artwork isn't too appealing, though. =\
The bad part is the Weakness, although we've seen much more radical ones. The first move may take advantage of it's weakness, but still doesn't make it less effective. This one is wide open for any Charizard/Infernape/Almost any other decent Fire-type Stage 2 to take it out.
9/10
Except no one in their right mind would use any of the Charizard cards and Infernape would have to be level X to guarantee a KO on a full HP Meganium. The fire types also can't have taken much damage unless they want to be knocked out by Fire Counterattack and either way, they always have the risk of being paralyzed by Ultra Power, which would be extremely bad for the opponent. And really, "Almost any decent Fire-type Stage 2" doesn't cover too many Pokémon and if they can't take out Meganium in a single blow before it gets to attack, they'll still be knocked out by any decent-powered attack after they're hit with Fire Counterattack. (unless your opponent has a quick way of healing their Pokémon)