I have used purifiers throughout the years. Does it work? Yes it does, but as to how much that benefits you will be another story. I had a friend with bad allergies that spend hundreds of bucks on one, didn't seem to make much of a difference for him. For me, it helps a lot with allergies, so I guess it depends on what you're allergic to in the air.
I have tried ionizers, HEPA filtered units, and combination units. The ionizers weren't very effective, but they were good to use in a room that requires quiet, and their main downside is that the dust, etc. that normally stays floating in the air lands on everything, so the room requires more frequent dusting.
The HEPA units are louder but way more effective. When you take out the filter for the first time and see how much crap is in there, it demonstrates how well it works. You can especially tell the difference if you have a fireplace and have to deal with smokey smells through the house.
There are combo units as well, which is what I use at home now to control the allergies I get from my cats.