Er, well they're really quite simple, as far as I recall. I never had any trouble with them when I raised them years ago (when they first came out). Since you have the kitty, it will only ever change into a cat, never anything else. In my opinion, they're pretty boring compared to Tamagotchis, but they're nice if you enjoy simple pets.
Yea, naNos are the easier electronic pets to take care of. Very simple. The only naNo pet that I had trouble with was the Teddy. The game was about impossible to win - so you could rarely make him happy. I always enjoyed the talking naNos. The bottoms responded very slowly though. The whole pet did... very slow.