As heating is the principal cause of dry air in the house during winter, I wish I had an in-line central humidifier. However, I have baseboard electric heating so it is not an option. But I did some cursory research last year when my sister was buying a house so this is what I can tell you about central units.
The Desert Spring was one of the better units I considered. I was also looking at the Air-King and Aprilaire (sp) units. Though it does not use a gunky drum, The Desert Spring appears to still use a standing pool of water which can cause a build-up of bacteria and mold. But it is connected to your water line which would seemingly mean that the water reservoir contains less water and is thus stagnant for a shorter period of time. I would opt for the auto-flush option to further the limit of use of water that has been sitting for extended periods of time. In addition, I would still occasionally clean the reservoir, if possible, with a mild bleach solution.
I think this particular unit injects steam or atomized water into the flow of warm air in the duct-work. I was told that the steam wand should not be installed too close to a bend in the ducts otherwise there could be condensation forming at the bend in your duct.
In cold northern regions these systems are great because heating units operate frequently and so humidity is also introduced frequently to the interior air. But in more temperate climates where furnaces run less frequently, there will be less "humidification" taking place because the humidifier works only when the furnace works. If a central blower is used solely for the purpose of humidifying the house, large amounts of electricity would be consumed just to circulate humidity (that is if your furnace can even operate the blower independent of the heating function). However, if you have a larger home, a central unit is the only way to avoid having many portable units operating at once (they can together can create a lot of background noise).
Finally, I try to keep my home at about 60% RH as that is most comfortable for my Ich. With a central unit that is difficult to do especially when room temperature is kept at a lower level, so you may still very likely have to supplement with room humidifiers. The benefit though is that the smaller room units will have to be replaced less often (mine break-down after about 2 years due to heavy use).
As I cannot have a whole-house unit, I use 2 larger capacity portable wick-humidifiers. I replace the wicks every year as that greatly increases the efficiency of the units. The wicks are anti-bacterial coated, but part of the reason I replace them each year is to further reduce mold or bacteria build-up. I have found that the best way to reduce interior dryness is to have a better insulated, more energy-efficient house. If you can, do something in that regard (install high R-Factor doors/Low-E windows, replace old chaulking and weather-stripping, better insulation) and the reduced operation of your heating unit will greatly reduce the dryness in the house. But nothing will eliminate the need for humidifiers for those of us with ich who have to contend with dry climates or seasons.