This. You should be able to unsubscribe at any time prior to a week before the next bundle (Grim Dawn) drops if you're on the month-to-month plan. If you did a 3/6/12 month plan then you're on the hook for the term you subscribed for, but even then you can still unsubscribe at any time and it won't renew at the time your term expires.
As for next month, I highly recommend Grim Dawn to anyone who likes ARPGs and doesn't yet own it. At $10.80 (new subscriber discount) you're getting a great deal on just Grim Dawn, so anything Humble pairs with it is a bonus. On the other hand, if you already own GD (like me), the most recent couple of months have not had as impressive a set of bundled games as previous months, so I am out once again.