After using Sharp's combination air purifier/humidifier, the Plasmacluster, for a couple months now, we're fans (no pun intended). While it's a little on the larger side for a tiny Manhattan apartment, with a true HEPA filter, a state-of-the-art ionization system and some choice extra features, we think it's worth it.
Using technology that automatically senses the air quality and adjusts fans, humidification and the degree of cleaning needed, the auto setting works really well and makes it easy to use. Lights indicate the different levels, which are a fun way to visualize the impurities and filtering action (green indicates clean) but can be turned off if you have other sources of entertainment. It also operates almost silently and cleaning it is extremely easy—a must for a daily-use device like this.
Buy it from Sharp starting at $400 for the smaller model.