I wouldn't consider any water adequate unless it has been tested prior to use. Properly distilled water can be very pure. That being said, there are compounds that readily co-distill with with water, making it impossible to remove via this process (there are some that won't be removed via a prior carbon filter, either). Reverse osmosis and deionization (together), alternatively, can make it possible to remove all those compounds. Volatiles also fit into this category, since the process of distillation can still incorporate them in the product water. Alone, the purity of distilled water vs. normal RO water is roughly the same (depending upon the distillation procedure and membrane). I suppose distillation could potentially be more effective here. But combine ultra-scouring resins after a membrane and there is no comparison. In the end, RO/DI is truly more effective at purification. It is also usually cheaper than distillation, although this depends upon the water being treated.