I've used everything, but MathScore (soon). While it seems counter-intuitive, rigor isn't among the first issues at looking at software - at least in a classroom environment. For example, a major concern is the number of questions in the database! If there aren't enough questions, then students cheat like crazy. Furthermore, the main value of online instruction - isn't the instruction - but the "continuous formative assessment."Thanks, OrangeMath!
Actually, good instruction is a problem. Several software vendors, such as ALEKS, intentionally keep the instruction brief, because it forces the students to think more in solving problems. The programs that really instruct well - they sell WELL to curriculum consultants, etc. - but students don't learn more. They do less work. This sounds bad, but consider textbooks. Students don't read them. They start on problems and flip to examples.
Much more can be said, but before developing opinions; use ALEKS first. It is by far the most robust product - with a solid AI engine - and it uses constructive responses. It may not cover enough for an independent active learner, if so; try Cognitive Tutor. If price is an issue and you work with primary SmartMath.eb.com and ixl.com are fruitful. However, for pre-K, start with Symphony Math.
If your focus is pre-Algebra, iPass from iLearn is actually a great product - a bit costly for me, and you need to buy into its 2x2, proportion approach.
With experience, you will develop bias that has nothing to do with Instruction. For example, StudyIsland has great questions and it allows users to submit corrections to questions (ALEKS has no wrong questions) but it's just multiple choice. APEX has great instructions and some constructive responses, but unlike ALEKS, the response must be exact in spelling, etc - a dumb system and APEX has numerous wrong/bad questions.
I haven't toyed with expensive solutions like Apangea. They include instruction with real person help. This is the big deal that many of you want. While Apangea uses US Math majors, the future is low cost help from India when a student gets stuck.
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OK, for those that want rigor, you should try Raffles from Heymath: http://heymath.com/main/productPrime.jsp. Heymath has excellent flash demonstrations combined with Raffles Singapore Curriculum. Spend some time on this site and see what is making Asia tick. Also, SmartMath is sold through Encyclopedia Brittanica, but it is Hong Kong's Planetii.
Math software is a worldwide competition with ALEKS and ST Math technology leaders, but there are many others. Remember, price matters when comparing.
Another key aspect of math software is "coverage:" not all students respond to a software program well. Every classroom needs at least two programs to reach everybody (time on task, number of problems). ALEKS has great coverage - somewhere between 60-70 % with my reluctant students, adding IXL or Smartmath gives me close to 100%.
Note: I laugh when people simply compare programs. As a teacher I play to find a fit. I need at least 2 programs. If you use just one, like "I Can Learn," which has nice embedded videos for instruction, I doubt if all students succeed. Don't be confused by averages in score growth, when every student must learn.
If I don't use ALEKS, I need 3 programs. For example, I respect Cognitive Tutor a great deal, but not many of my students respond to it. They quit.