Too much moisture in the wrong place means mold! Not enough in the winter means dry skin and mucus membranes. I recommend individual room ones rather than the whole-house variety so there is no mold accumulating in ducts where we can't see it.
Having Scottish blood I have tried cheap ones: the atomizers and the steam variety. These either wet the floor or require cleaning with vinegar everyday, respectively. This Honeywell model uses ultrasonic sound and requires no maintenance other than wiping out minerals occasionally. Output is also adjustable. And for ten more bucks, versus other similar models, it is rather attractive. Click on image for link.