Yes, they act as a single character-especially if you have an only child, which IS possible. They also do many unique things-all improvements. If I had to list all the downsides, I could do it now.
1) You can't connect to previous versions
2) You have more trouble to check if the baby is a boy or girl because the babies (Mimifuwatchi, Omututchi, Futabatchi) are only in a single color, unlike previous versions.
Upsides, on the other hand, would probably take hours, so I'm not doing it.
A Familitchi IS worth getting, but it's kinda complicated for new users of tamagotchis. I recommend reading some logs or topics about it, if you really want it, to get to know it a little better.