Dual Hearts is very much an early 3D-Zelda for the PlayStation 2. It features non-linear exploration, somewhat linear progression, puzzles, dungeons, bonus objectives and activities that can upgrade your character, various weapons, etc. While the graphics are not exactly *good*, they are charming and highly reminiscent of Dreamcast titles. It's very colorful and pleasing to look at. Music is also fairly underrated - you have the sunny, tropical paradise themed OST in the hub world, but varying genres while in dungeons. They all sound like midis so don't expect masterpieces, but they are mostly memorable boppers.
The gameplay is split up into two sections: hubworld and dungeons. Most of your gameplay will likely be in dungeons, but you can spend some time in the hubworld figuring out what to do next or to accomplish some other tasks for personal growth. In Dual Hearts, you invade people's dreams (while they are sleeping) and those dreams are transformed into dungeons by the person's twisted desires or regrets, brought on by emotional baggage. In these dungeons, you'll be fighting enemies and solving puzzles. Weapons are gained at a reasonable rate, with all having different functions and properties that you will be repeatedly using across all the dungeons in the game. You also gain experience for leveling up those weapons by killing enemies with them, but the game doesn't explicitly say this. I recommend leveling up all weapons as it unlocks additional functionalities/moves and makes them stronger.
While the combat and graphics are serviceable and everything else is "good", the enjoyment of Dual Hearts comes from the excessive charm. The interactions between Rumble and Tumble (your dream-pig best friend that can speak, by the way) are entertaining, and the dialogue between all the characters is also like this. I think this is a very good example of early Japanese localization, although there is no voice-acting.
I really recommend Dual Hearts to anyone that hasn't played it yet and if they are looking for an early 3D, but very charming Zelda-like. Unfortunately, finding a copy of Dual Hearts (in good condition) will run you upwards of 60, maybe even 100+ bucks for sealed copies on eBay. Dual Hearts DOES have a digital release, but ONLY on the PS3/4? in Japan, and no other country. I have asked Atlus support staff why it is only available in Japan - essentially, the English localization rights belong to Atlus JPN, and it's up to Atlus JPN only if they want to release the English version again... but it's caught up in licensing issues. I highly doubt Atlus will ever think about releasing Dual Hearts (ENG) ever again, unfortunately. However, I have heard from a friend from a friend that Dual Hearts now works perfectly in a certain emulator and even has a working widescreen hack...