When it comes to cleaning, a painting knife has a decided advantage over a brush. Wipe any excess paint off with a cloth, then wipe the knife again with a clean cloth. Often that's all you'll need to do to clean a painting knife.
If paint has dried on the knife, I usually scratch it off using a damp cloth and another knife, or sometimes take a nailbrush to it. But fundamentally the paint will usually scrape off like from a glass palette.
If you're lazy about cleaning your tools, a painting knife is definitely more forgiving than a brush, as a little dried paint on it has less of an effect than dried paint in a brush's bristles. However, if a knife is made from steel (rather than stainless steel), it may rust if neglected. The knife in the middle of the photo, for instance, rusted after being used for a series of plein-air seascapes and never cleaned after exposure to sea mist.
If the blade of a painting knife ever breaks (though it'll take some abuse to do so), don't throw it away. First see if you can't smooth the edge down into a new knife, even if it's one suitable only for scraping.
