It isn't terrible, but it's not great either.
The Bad: Multiplayer requires two cards; basic gameplay is too simple; collecting is monotonous and gets old; boss battles are too easy; you're a stinking slime!
DQH: Rocket Slime is a great game for little kids or the braindead. It's very hard to get into because it's so repetitive and easy. It's got charm to it, sure, but really, the gameplay's just okay, and it's not very fun to play for very long. Some people may enjoy its simplicity, but it's ultimately not great.
In Rocket Slime, you are cast as Rocket, a...well, a blue slime, which, in case you're unfamiliar, is the lowest and simplest enemy in the Dragon Quest universe. It is akin to the goombas in Super Mario Bros. If that seems like an odd decision for a main character, you're right - it doesn't make for a particularly thrilling game. Rocket has exactly one attack, and it's not terribly exciting either.
At its heart, Rocket Slime is a gathering adventure. It's all about finding items, locked up slimes, more ammo for your tank...yeah, it's all been done before. What makes RS passable is that it does it well enough, and it has enough spunk to make it mildly interesting.
The tank battles are pretty much the only notable part of Rocket Slime. In these epic battles, you fire projectiles you have stored up at your enemy tank and keep the enemies from busting up your tank. This is a rather amusing exercise, though a bit tedious. On a side note, multiplayer tank battles can be enjoyed between two people who both own the game.
Overall, Rocket Slime doesn't have a whole lot going for it. It looks and sounds nice enough, but it just doesn't use the DS features to its potential, and it comes off being more suited for a young crowd that doesn't mind repetition.