Alright, here's what I've been told. I considered getting one a while back, but there are two things that stopped me.
1) I have carpeting in my home
2) I have 2 cats
If you have carpeting and/or any type of pet that sheds, I've been told by my doctors that an air purifier is practically pointless because the germs and other air-borne bacteria will just continue to be around in the carpet and on the fur of the animals. Other than that, if you don't have those things, I say go for it!