There are a few different types, warm mist, cool, and ultra sonic. I personally use a ultra sonic. I've tried many in the past and most cool mist styles use filters, which can get expensive. Warm mist styles use more energy to run and depending on the water quality at your home require a lot more cleaning to the heating element.

The water at your home will also play a roll in how often you have to clean the unit.

I use a Holmes HM2610-LUM and have never had a problem, very simple design, easy to clean, and lasts almost 30 hours on low. The low setting keeps my 12x12 room at 55-60%.