As with any system in which you borrow money, Affirm is only a good idea if you are financially stable enough to afford all due payments on time. Zero-APR loans are the best choice when selecting an Affirm repayment plan, as you won't owe additional interest.
Affirm may not be the best option for those who wish to grow credit as it isn't consistent in reporting to credit bureaus. When Affirm does report, it reports only to Experian.