This game would have been banned under Soviet Communism.
What would you do when some old guy with a head shaped like legal tender appears in front of you and tells you that you can reach heaven by throwing huge amounts of money into the pond near your house? That's your mission statement in this game: Make money. For that purpose you are turned into Tingle and sent out into the world to make money. You can interact with NPCs (who almost certainly want money), fight monsters (who consume money when they hit you), solve dungeons (which charge an entry fee) and do many other money-related activities. If this confuses you, it needs to be made clear that Rupeeland is still close to the Zelda gameplay: You walk around on an overworld, occassionally enter dungeons full of monsters and puzzles, fight things and explore to find hidden treasures. Except instead of just having to find x special things you also have to find a huge amount of money to progress (technically you only need the money but quests are just the fastest way of getting money).
The overworld is mostly a map divided into several square sectors (like the screens in a Zelda game except larger than a screen) with some mostly easy terrain (there's rarely much tool usage to get through an area) and of course a random scattering of enemies. Enemies aren't just obstacles like in Zelda, they're also prey like in an MMORPG. Killing a monster usually makes it drop materials or valuables. The former is used in cooking to create valuable or useful items, the latter is sold for money. Fighting is done by simply running into an enemy (well, 99% of the time, occassionally an enemy needs other strategies) which creates a dust cloud containing the fight. You periodically lose money (as it's your health, if you lose all money you die and have to restart from your last save, no "save & continue" here) and after a while the fight ends, if you're successful the enemy disappears and its loot drops on the ground to be picked up. While the dust cloud is there you can move and hoover up other enemies which greatly increases the loot (one enemy drops one item, two drop three items, ...). You can also tap the cloud rapidly to fight better. And finally you can hire bodyguards to help you fight, when they're part of the dust cloud you take less damage and the enemies die faster. You know what? Scratch that. It's not "you can hire" but "you must hire". Fighting alone is unprofitable since you take a lot of damage and have low chances of success (except during the very early game). Bodyguards have a heart meter that depletes during fights (slower if they're fighting with you instead of alone), once it's empty the bodyguard quits his job and has to be rehired (he remains in place so you only have to pay, not get back to the job agency). You can heal the bodyguard with medicine you can brew.
Dungeons are like a dungeon in Zelda, they contain puzzles (though they are simpler than Zelda dungeons) and enemies, enemies only drop money (and not enough to be profitable). At the end of a dungeon awaits a boss that is fought in a minigame (e.g. shmup, punch-out, ...) and if you're victorious you get a huge amount of money. Dungeons appear roughly every second area in the game.
Apropos area, as said above progress is gained with money. Specifically, throwing money into the pond next to your house makes a tower grow and with each growth step you can reach a new area on the overworld (the areas get larger as the game progresses). Most of the time doing all the quests and dungeons in an area will get you enough money to grow the tower, if not you're usually only a small bit off. When that happens you're in a bad situation though, grinding for money isn't fun.
The much talked about haggling is fairly simple, you input a number and if it's high/low enough you win. When paying you have to guess the amount the NPC wants (sometimes there's an indicator that shows a value much lower than the asked price), you pay what you name but most of the time that money is wasted if you paid too little. Some add the money up to a total but most don't. When you get paid you similarily name a price and if it's too high get told off, depending on the situation you might get multiple attempts. Rewards are larger than payments, don't be afraid to ask for seemingly huge amounts of money, the reward values are based on your progress in the game, not the difficulty of the quest! Sometimes you get fixed rewards, I'd estimate there's always at least one quest that gives a fixed amount per area to give you some base income. You'll get a feeling for values sooner or later even though it may seem daunting at first (this could really have used a tutorial with some example values).
Exploration gives a bit of money as well, when you see landmarks that aren't part of the map you bought (well, if you have one) you can draw them in by drawing a circle around them on the map and you can often tap them or otherwise interact to get some random items (rarely valuable). The circle has some issues, for one it doesn't always register as closed and you'll need some practice before you draw them right and the other is that the landmark's physical position doesn't always match the map position. Just because you can't draw it doesn't mean it's not a landmark! Try drawing the circle larger (should always be pretty large) and offset it slightly in some direction. Mapping landmarks gets you some extra cash, it's worth doing. Once you complete a map you sell it and have to rebuy it at a higher price so be careful!
Saving is limited in this game, you can only save at your home! Dungeons have quick exits if you have a certain item but no quick return to where you left off. I haven't tried playing the game on the commute but I can't imagine that to work well, you'll have to suspend without saving fairly often if you do anything that takes more time. Fortunately you can return to your home instantly when you're on the overworld so returning should always be doable within maybe 5 minutes of deciding you want to quit (which is still way too long for a handheld game) but playing in bursts won't let you get much done.
Overall the game is fun except for the moments when I had to grind (which fortunately were only 2-3 and never very long while I'm pretty close to the end now). I'm not sure it's a very portable game though. On the upside at least I never had to use the microphone but the dpad+stylus control is pretty tiring for the arms. It does manage to make you feel attached to your money though and unlike, say, Twilight Princess you never feel like you have no use for all that money.
Recommendation: All in all it's worth a buy. It's arguable whether you should get it at full price but it can be worth it if you like the gameplay. I'd stay away if you play your DS primarily on a short commute, Tingle should be played at home.