I have three in total and two different models.
We have a BlueAir 203 for my daughters room and two Sharp PLasmacluster FPA80 in the guest room and master.
The problem, as stated by xfactor9, is that finding filters can be a problem on certain models. A ton of the Asian brands, i.e. RabbitAir, Winix, and CoWay (who makes models for other companies like RabbitAir) can be problematic in that they change models often while not keeping the same filter format. Sharp has done this as well. The newer model, like the one I have, seems to have a standard format that they are using on some of their other models so maybe they have fixed this issue? Some brands like Austin Air, IQAir, BlueAir etc are really good at product support and filter replacements. You can get more technology for your money with the Asian brands, just know that filter replacement and support can be hit or miss. The others like Austin Air..etc typically have a higher initial cost, but with better support. This is what I have experienced personally and is not me saying this is the way the air purifier industry is. Your experience could be completely different.
Getting a good one is the best bet for noticing anything. Some people don't notice anything at all, while others, me included, notice it immediately. The question is what are you trying to clean? Odor? Allergens? Smoke? The answer to these question will steer you to a appropriate brand and model.