I do recommend using distilled water. Even though the steamer can use tap water, solids in the water can cake on the heating element and periodically vinegar needs to be ran through the steamer and for the cost of vinegar, it's jsut as cheap to use distilled water. Just my two cents.
Removing roller pen and crayon stains? Yes, it does remove ink and crayon stains. It's almsot cathartic to have your little icasso's art work disappear from view! Larry has some very good videos showing how to best used the steamer.
Yes, it cleans floors and windows, aticualary in the corners like nobody's business. That keeps the spouse happy.
You might think about keeping a couple of detailing brushes for the engine compartment separate and the brushed used for the interior. This way, no stray oil makes its way inside the car.
Again, it's not cheap but this is a forever machine. It's nice to have a "Porsche" without paying "Rolls Royce" prices. Serioulsy, thresholds, brake calipers, wheels, rubber molding, crevices oare exterior lights, MBtex, leather, carpets, oil lines and fuel lines are all fair game with this tool.. Think of Snap-on as to quality.