So far I have gotten acquainted with Peaches, Rags and Buddha. I think the Imaginary friends are great when the kids are still toddlers and the household doesn't have enough funds for toys to amuse the children while the adults are busy. It also helps the toddlers gain a +20 moodlet for playing with the Imaginary Friend. When the toddlers age up to Child however, there are a lot more things to do and usually there will be real friends to make so my Sims don't play with them anymore. I also tried putting them to the ground so they could walk around the house a bit but the interactions are meant to be a playmate and helpmate of the Sim child. Some of the interactions, I think, are kind of mean and maybe good if the Sim child is evil.
If the time permits and if the resources abound, I usually turn these faithful fellows into real children. It's just like adopting a kid because you don't get to choose the traits. I love playing big families anyway so another child is awesome.