Dueling Dubs: For some reason, the games are almost always given two English dubs. One for North America where the characters all have American accents, and another for the UK where everyone has a British accent.
Piposaru 2001, Million Monkeys, Piposaru Racer, and SaruSaru Big Mission were the only titles in the series to not get an English version. An Eyetoy entry in the series, Monkey Mania, never got an North American version due to poor reception in Japan, Europe, and Australia. Ape Academy 2 similarly only got released in Europe and Japan. In an odd case, Pumped and Primed was not released in Europe.
Also the anime series, Saru Getchu -On Air-, hasn't aired outside of Japan.
The PlayStation Move reboot is available in North America. However, if you want a physical copy, you have to get it from Japan or Europe, because the North American version is download only. Same goes for Ape Quest, except Europe didn't get a physical version.
The games (at least, the North American versions) are inconsistent with the voice actors. This is bad enough that even the PlayStation Portable port of the original had different voice actors from the original PlayStation version.
Solid Snake's guest appearance in Ape Escape 3 was voiced by Peter Lurie (who voiced Vulcan Raven in the original Metal Gear Solid) instead of his usual actor David Hayter.
What Could Have Been: The Nightmare Scenario Monkey Fable from the 2nd game was supposedly the original idea for the game.