It's not perfect, though, and my experience differed a little:
> MathAcademy excels at identifying exactly what you need to work on, zeroing in on your weak spots and areas for improvement.
It doesn't excel at this yet. It claims to have an "adaptive diagnostic assessment [that] will identify the correct starting place for the student" but this didn't work very well for me. I ended up sitting through many questions that felt insultingly easy (e.g. number lines) without a way to skip them. You have to answer the questions quickly and correctly to prove you understand them, which is probably a defence against students clicking "I understand this" for everything when they don't, but it felt very tedious.
If you get a lot of questions wrong on a subject it ends the topic early with a (somewhat discouraging), "ending your session early due to poor performance" (why not just, "let's come back to this later"?). But if you get lots of questions right quickly it doesn't seem to have a similar, "lesson ended early due to great performance" or "ok, it's clear you already understand this" — it just continues to drill you, which feels very tedious.
> For less than the cost of a single hour with a private math tutor, MathAcademy provides you with 24/7 access to what is essentially a tireless, expert math tutor.
It's less like a tutor and more like a choose-your-own-adventure textbook with tests, spaced repetition and guided pathfinding. And that's great! But I missed things I'd get from a tutor, like the ability to ask for clarification. MathAcademy uses terms in questions or descriptions that it hasn't defined or explained previously (or perhaps it had defined those terms in earlier lessons it skipped?). I ended up looking things up from outside sources or taking a 30-minute detour into YouTube to fill in the gaps. Which is fine when it's occasional, but annoying when it's every other session. Sure, it's "adaptive" but it's not responsive like a real tutor.
The overall tone is that of the forever unimpressed tutor — quick to penalise and very light on encouragement. Full gamification with gold stars and badges and weapons upgrades would be annoying, but I'd like something more than "you scored x". (For example, immediately show me how my recent result has affected my overall understanding of the subject as a progress bar.)
The UX of the site is tedious when drilling questions daily. The slow round-trip before seeing if your answer was correct becomes grating; if this can't be done client-side I would love to see improved performance server-side.
On the whole, though, yes, it's worth checking out if you're serious about maths improvement.