[Long winded response!]
Although I do enjoy Sage, I have to agree with others that it is not quite user-friendly enough for high schoolers to tolerate just yet. I would also echo those that discount W|A due to it not being free. Maxima being on smartphones interests me, but I offer another alternative.
The free dynamic geometry software, Geogebra, is perfect for high schoolers. It has a strong (and intuitive) CAS to be able to do anything they'll want to do and combines the power of geometry with algebra, as should be happening in the classes. They're working on phone/tablet versions (just released). The commands for things are simple, and the software helps guide you for input if you're stuck. Please check it out! [Don't use the browser version.]
The question of whether to use these tools or not... I'm kind of surprised by many answers here. To not introduce mathematics students in this century to technology and have them actively using it is just shocking and a great disservice. We should be embracing these tools to provide more legitimate mathematics to our students. Let's take an example.
A cookbook problem that exists in a class with no calculators/software could be something (for example) like:
Solve $x^2-5x+6 = 0 $. And maybe a few of these are necessary to get a feel. However, what is ventured by spending days doing these things by hand with ever more ridiculous problems like $2415x^2+433x-20=0$? That is not a real problem. It comes from nowhere, and no one does these kinds of things without software -- neither mathematician nor applied scientist.
A real problem might result in the cubic $−0.0080556t^3 + 0.11881t^2 − 0.30671t + 3.36$ (from here, example 1) where we are taking data and fitting a curve. Now I might ask (because this came from data about gasoline prices over time) when the prices were increasing. I can ask that of someone in Algebra 2! Even in Calculus if this is asked, a calculator/software is going to be REQUIRED to find derivatives and arrive at actual numbers. Before Calculus, they can use technology to firstly -- find that cubic(!) with the given data and then discuss what it means.
[Point of honesty: Of course I was taught the cookbook stuff! I thought it was awesome! Let's simplify rational expressions and verify trig identities all day long, in my opinion. But I'm a mathy, nerdy teacher. That just isn't doing (real!) mathematics nor applications justice.]
And to finish my rant, I would argue that the technology can more quickly allow teachers to introduce students to conjecturing about very hard things more readily than if they don't have such access. Please see the journal "Mathematics Teacher" for more cogent and detailed arguments in this vain.