How easy is it for a student to find the calculation/function they are looking for? The emphasis here is on working out how to do something without any help from someone (teacher/peer) or something (manual): Experimentation, and working through options from most likely to least likely are some of the key strategies. 

Each video below runs through, putting myself in the shoes of a first-time user, a concrete example of "how do I work out x,y or z"? 

Challenge1: Two variable statistic calculation - 1st: HP, 2nd: Casio, 3rd: TiNspire

You may disagree with the above ranking! Watch the video below and give your justifications in the "comments" section below to help the whole community develop a deeper appreciation of each model [as with students, the aim of sharing and discussion is to move towards the optimal solution, rather than 'show the group who's boss'! - real 'bosses' don't need to 'show it', it shines through ;) ]. 

Challenge2: Scatter Graph and Linear Least Squares Regression 1st Casio, 2nd TiNspire (but great for investigating the concept)

Challenge3: Functions: roots(zeros/x-intercepts),Maximums+Minimums: 1st Casio (only just), 2nd TiNspire, 3rd Ti84 Plus (by a distance)

Challenge4: Intersection of two graphs/functions or Equations Solvers