Simple and repetitive, but if you like the formula the gameplay that's not a bad thing.
The gameplay is really very simple, most of it is made up of finding certain objects within a large scene filled with a ton of out-of-place clutter and random things. For every person you investigate, a certain number of items must be found to progress, and those items are spread across multiple scenes. As you advance, you'll gain access to such items as a flashlight for finding things in the dark, an x-ray device for seeing through objects, goggles for seeing through water, and a 'super straw' for blowing away debris via the DS's microphone.
Overall it's a very simple game that doesn't require a whole lot of thought. Some will find it boring, I'm sure, but if you like simple find-the-object games, you'll like MillionHeir, for it certainly is one of the flashiest and high quality ones to date.