It's pretty needy as far as Tamagotchis go. I'd put it in second place behind the Mori no in that category, behind the Umi no. Luckily it doesn't get attacked by predators like the Umi no, but it's still not one you want to leave on its own. It just loses hungry and happiness hearts too quickly for that.
Beyond that, it's also rather long-lived and takes a while for it to change between stages.
Its beep is kind of loud; I found it waking me up any time that I'd forgotten to turn the sound off overnight. It also has an odd 'double beep' for everything; it's a little hard to describe, actually. It's kind of musical, though, and the game music has a really nice beat to it.
The game is the same as the one in the Mothra Tamagotchi and the Mori no, you just have to guess which of four holes the item is in, bones in this case. It could just be me, but I think the game feels a little harder than in those other versions.
It only had one shell color, brown with bones on the outside and red trim around the screen. It's cute, but it says 'dog' to me more than it does 'caveman.'
The typical discipline meter has been replaced with an evolution meter that helps effect what your pet will change into. When you see the Genjintch making pottery, you have to use the praise icon, which replaces the discipline icon, in order to raise this meter.
There are several characters available, based on things like monkeys, cave-people, aliens, and even a mammoth. I've read the Ginjintch can potentially revert to previous stages if neglected too much, but I haven't seen this happen myself.
Rather than ending with a death screen, the good ending has your Genjintch taking off in a rocket.
The Genjintch is an interesting Tamagotchi. The characters are long-lived and needy, so it may not be one you want to get unless you have a lot of time to spend with it.