Seems like a good plugin so far. I like the fact it is simple and free! (Given that Akismet can be a bit expensive now).
A couple of basic suggestions for improvement: the maths check is placed *after* the submit button, which is not great for usability. Far better if it could come before the submit button. I know this is probably achievable if you implement a custom comments template, but if you don't have that, it seems hard to do.
Also, the language of the default instructions said "the following maths question" when the maths question was above it – you might want to tweak that. (Again, I know this one is very easy for us to tweak by editing the message too).