Pet insurance can be a difficult subject to tackle - primarily because, generally speaking, it doesn't work like human insurance. For example, typically, pet insurance is completely reimbursement based. The pet owner takes their cat to the vet, encounters a covered expense, and then pays for it in full at the time of service. The pet owner must then file a claim and wait for reimbursement.
From that point forward, some companies work a little different from others (similar to how some health care insurance carriers cover different things, at different rates and deductibles, than other carriers). One thing that is for certain is no carrier is going to cover a pre-existing condition and most of them have sweeping language that broadly defines what a pre-existing condition might be. Typically this is going to be any illness, injury, or condition that existed prior to a certain number of days before purchasing the policy (sometimes as long as 2 years). Thus, if your kitty was taking methimazole for hyperthyroidism prior to purchasing a plan, it is unlikely the plan is going to cover that medication or examinations for re-checks on that condition. If you purchase a plan for your kitty when he or she is young, and that kitty develops pancreatitis several years later, that would probably be a covered condition.
Pet insurance is an excellent option for pet owners who might not be able to afford a certain level of care in the event of an emergency or serious illness. It is unlikely an insurance purchaser would receive more in benefits than what they paid in (otherwise, how does an insurance carrier make money?); however, having the peace of mind in knowing you could provide the best care for your kitty in the event of an emergency might be worth consistent, flat, monthly payments to a pet owner.
For more information on pet insurance please read the article I used as my source on this topic from Reviews.com. BestCompany.com also has reviews of several pet insurance carriers.