ScummVM can be used to play various versions of KQ1-KQ6, as well as many other Sierra games using AGI or SCI engines. Including those across different platforms (PC/Tandy/Mac/Apple II/Amiga/PCjr).
ScummVM adds mouse control to the SCI games, giving a feel much like the apple II mouse control
In the AGI games, ScummVM changes the colors of KQ2 Apple IIGS from Apple IIGS colors to PC/Tandy colors or Amiga colors. For example the colors are richer, Graham's clothes appear red, as opposed to pink (it's even more noticeable in Space Quest I, where Kerona's sky goes from orange to purple!). In addition, the interface appears to be that of the PC/Tandy/PCjr saving system, as opposed to the Apple IIGS interface. The latest Beta version 1.4 are capable of playing Apple II sierra games sound almost flawlessly. So while it may not have an authentic emulation of KQ2 from Apple II standpoint, it might be noted to be the best of both worlds (combining the original PC colors with the improved sound of the Apple II)!
The emulation of Apple II version of KQ3 adds in the timer that was missing in Apple 2E/2C (but not Apple IIGS).
ScummVM can access PCjr/Tandy, etc, sound systems that may have extra channels, such as sound effects for crashing waves, not found on the PC Speaker versions of the games. In theory the cricket chirps should be heard in KQ1 PCjr version.
ScummVM is able to access the Enhanced version of KQ6, although it uses a hybrid of Dos cursors (color)/hires artwork, as opposed to the black and white cursors in windows.
The save system in many of the games have replaced with a ScummVM saving interface (though this can be changed in ScummVM's settings to go back to the classic game interfaces).
ScummVM currently cannot play the Amiga version of KQ6 (as it is not based on the original SCI system, and instead uses the Virtual Theatre engine). Although versions of the Virtual Theatre engine used on other games have been implemented, the version used in KQ6 Amiga has not been emulated as of yet.