I've had a Pocket Neopet, and hated it. It had the same life span as Giga Pets (about 15 days) and it was soooo hard to keep happy! You have to feed it at certain times, and when you did, your pet would STILL lose health points. The Pant Devil randomly comes and steals items, which is mega annoying. I had a petpet that took me forever to get and guess what? The pant devil stole it! And you have to play games all the time to work up heaps of money, because you have to buy stuff to care for your pet. Four of the six games can only be played 3 times in one day to earn money (well, what I mean is that you can play as much as you want, but you can only earn money three times so make the most of them). The other two are gambling games and they're REALLY annoying.
My dad has this robot made from the components sold with the Real Robots magazine. You can build a handset for the robot too, and the handset can be used to control the robot, or to play games... And one of the games is a virtual pet. A little boring at times, and takes forever to evolve, but somewhat nice to have around. Too bad I don't have it anymore
And I have a Littlest Pet Shop thing as well. It's mostly based on the games you play to earn stamps and stickers. There's a pet you have to care for, but you don't really have to care for it as it never dies. If you don't brush it and play with it regularly though, it becomes disobedient and won't do as you ask, so you have to brush and play with it. If you don't feed it, it moves very slowly and takes a longer time to do stuff. Again it's nice to have around.