Like, when |I buy it today, can I cancel it after if I don't think it's worth it?

You can whenever you want, although if you cancel it the membership ends immediately (you lose animations and chaos, but keep the coins). Ex. You buy chaos on January 1, then you cancel it on January 2nd, the membership ends on January 2nd, even though you paid for 1 month.

Why would you do that, anyway? I mean, why not wait until membership ends?

Would be much better, IMO. Atleast try out Chaos. After trying it out, myself, I personally prefer Order, but it's still great if you know your tactics.