Well, the short answer is, "it depends." Ask yourself these questions:
What is included?
If the contract is very cheap, it may be just a ploy to get a technician into your house and try to sell you something you don't need. However, if the contract entitles you to a discount on parts or labor, or priority service or emergency/after-hours support, then it might be very well worth it! If a system is going to break down, it will typically do so at extreme temperatures: the air conditioning will fail on a very hot, humid day, or the heat will fail during a prolonged stretch of frigid weather. These are times when you can ill afford to be without a working system and when all HVAC companies are inundated with calls. Without having a company that will put you to the front of the line, you might find the wait with any company you call will be measured in days, not hours! Additionally, most HVAC companies will charge extra for after-hours support. You could wind up paying up to twice the labor charges for getting your system serviced on a Saturday versus getting it serviced on Monday. But in extreme temperatures, you may not be able to wait until Monday! So, if your service contract entitles you to no additional up-charges for after-hours emergencies, the savings in the labor on just one visit could be worth it.
How old is your unit?
If you have a brand new unit (less than 6 years old), the chances of it breaking down might be less than if you have a unit that is more than 10 years old. Also, if the unit is still under warranty, then it could be that some portion of parts and/or labor may be covered by the warranty. But be aware that manufacturers may not honor the warranty if they think the unit has not been properly maintained by the homeowner. Having a service contract where you can prove the unit had regular preventive maintenance might make all the difference. Certainly, if you have an older unit, or a unit with a history of previous failures, having a service contract might be a smart move!
How diligent are you in doing preventive maintenance?
If you're following the tips we provide on our HVAC News page, videos, and in our Fall Maintenance eBook, and are diligent about changing filters on time, cleaning the coils, keeping the condensate line clear, and doing other recommended tasks yourself, then you might not gain that much from having a service contract, especially if it only includes these same types of services. Of course, some preventive maintenance tasks require special equipment, measuring devices, certifications or licensures (such as adding refrigerant to a unit), so there's no such thing as never needing service. But if you're pretty handy and diligent about doing all you can, you certainly will lessen the chances of some things going wrong.
Consider Potential Savings
Annual maintenance is vital to keep your HVAC system in good working condition and at peak efficiency, so you'll benefit by (a) avoiding big repairs bills, since minor problems could be spotted before they escalate, (b) lowering your utility bills, since your unit will be running more efficiently, and (c) extending the life of your system, so purchasing a new system can be delayed. Like your car, it is cheaper to maintain it in good working order than to deal with large repair bills when it breaks down.
Read the Fine Print
Always read the contract, so you understand exactly what's included, and what's not. For example, if you purchased a service contract from the gas company thinking it will cover your furnace maintenance, you may find it only covers repairs to the gas lines from the street up to the furnace; the gas company won't be coming out twice a year for tune-ups and regular preventive maintenance on your HVAC unit.
Consider the Company, Not Just the Price
Remember, the service contract is only as good as the service provider backing it up. Are they honest, trustworthy, and reputable? Are their technicians trained, certified, licensed and insured? Do they have experience with the type of equipment you have? Do they treat their customers with respect? Do they have good online reviews by homeowners (or business owners) and by the Better Business Bureau (BBB)? Have they been in business a long time? If you base your service decisions solely on price, you may not be getting a good value for your money.