Product review (5 stars): I don't know if I love it, but you really need it.
It's a bit expensive, and it may look like Venta's just trying to sell you a questionable extra, but don't try to run your Venta Airwasher without it. It acts as a wetting agent, among its other more subtle effects, to utilize the full area of the machine's rotating plates. Without it, water will only appear on the plates in droplets. The smaller area of exposed water surface will result in much lower evaporation efficiency and disappointing humidifier performance. That will contribute to scale accumulation on the plates and inside surface of the tank, as well, further reducing the machine's efficiency and leading eventually to a difficult cleaning process.
I'm less certain about the need for Venta's cleaning solution, also expensive. I pretreat the water I put in the Venta to zero ppm dissolved solids (see my review of the ZeroWater ZP-010 10 Cup Pitcher), thereby eliminating scale accumulation. Apart from that, others have reported being able to clean their Airwashers with simple vinegar, which is a mild acid that dissolves scale. It may just be slower than the proprietary cleaning solution.