If you're coming from a phone like the Galaxy Note 4 and Galaxy S5, you'll probably be able to acclimate to the Galaxy S6 and its software fairly quickly. TouchWiz has changed (for the better) and you'll be familiar with many of its features and their names.
If you aren't coming from a Galaxy phone, you're probably going to hate the Samsung Galaxy S6's steep learning curve. And yes we know there is an Easy Mode but the fact that there is an Easy Mode helps to prove our point.
Samsung's Galaxy S6 comes loaded with features but many of these features are hard to find. Samsung simply doesn't do a good enough job explaining them to new and novice users and it could be months, perhaps even years before you're able to unlock the phone's full potential.
The Galaxy S6 is easier to use than previous Galaxy S models and the Galaxy Note but it's still not as intuiative as the iPhone and iOS. iOS is much easier to pick up and master.