The number of buttons/knobs on the APC40 is great.
For Ableton: I can't think of another controller that gives you as much functionality.
For Traktor: I use it to complement a VCI-100. I like the traditional layout of the VCI-100 and use it to EQ/mix/browse and load tracks. The APC40 gives me access to pretty much every other feature in Traktor without having to use a lot of modifiers and double assigning buttons.
Nevertheless, the APC40 can function very well as an all-in-one controller.
Bookmarks