Air purifiers comes in all configurations, sizes, and feature sets. Their are also a number of brands on the market ranging from some well-known to lesser known brands like IQAir, Allerair, Blueair, Air-O-Swiss, and Friedrich to name a few of the more commonly known ones.
A common reason for having an air purifier is to help alleviate the effect of allergy season on those who suffer from allergy symptoms yearly- whether to animal dander, or things like dust mites and bacteria.
Another common reason for having an air purifier is because a member of the household smokes -- certain air purifiers will help mitigate the problem by shrinking the amount of smoke particles in the air. This also helps reduce smoke caused by cooking, steaming, or from barbecuing.
One big consideration for your choice of an air purifier has to do with the ongoing sustainment of it. In general the heavier the air purifier system such as those unified into the central air cleaning system will necessitate more sophisticated methods of maintenance.
Other more simpler models such as stand alone air purifiers are comparatively easy to clean and maintain. Filterless air purifiers especially make owning one of these cleaning devices a cinch. You can derive the benefits of air purification without shelling out lots of dough or trouble about maintenance costs both in terms of money and time.
In the end the choice for the right air purifier is highly subjective and thus should be sought out from the perspective of the consumer's needs and wants. Up until this point there is no real one-size-fits-all solution that will make everyone equally happy. First make an inventory of your most pressing needs, and prioritize from highest to lowest.
This will make your purchase decision much easier when you do go shopping for an air purifier.