It still is immensely popular in the Czech Republic. It keeps appearing approximately once a year. New generations of children fall in love with the show, and adults watch it with nostalgia goggles. It helps that the voice cast is made of famous respectable actors.
Also in Hungary, especially after the public service station M2 got re-branded as a kids' channel and started airing the series again. It's mostly remembered for the same reasons as in the Czech Republic, and people often bring it up when talking about "those good old cartoon shows".
Also amazingly popular in Finland.
Memetic Mutation: In Spain there is a slang idiom made after this show: "¡Tienes más peligro que Willy Fog con el abono transporte!". Which roughly translates to: "You're more dangerous than Willy Fog with a public transport pass!"