If you're already saving for retirement you can compare your savings plan with KiwiSaver before you decide whether to join.
If your existing savings plan is a complying fund its benefits and rules are similar to KiwiSaver's.
Ask your provider if you do not know if your scheme is a complying fund.
If your employer makes contributions to your other scheme, this can affect the amount of compulsory employer contribution they are required to pay, so you may not be entitled to another employer contribution to your KiwiSaver scheme.
Ask your employer or existing superannuation provider for more information, and consider discussing your options with a financial advisor.