It may be if: Your system is more than 8 years old.
Systems of this age and even newer are not nearly as energy efficient as current systems. Unless you purchased what at the time would have been considered a ultra high efficiency system, you are losing money every year you keep it. Consider a 10 SEER system versus a base model 13 SEER system of today.(that's as low as it goes now) Are you really happy to spend 30% more to cool your home?
You have been experiencing reliability problems.
It just won't stay fixed!! Today's air conditioning systems have some really good warranties, even on entry level models. Most have 10 year compressor warranties and that's a big ticket item. Five year for all other parts is usually the minimum and on even slightly higher price models it goes to 12 years. If you have a compressor failure within the warranty period you can choose to have the complete outdoor section of your system replaced at no charge, except labor. Labor warranties are available as well.
By combining the cost of operating your old system, the cost of repairs, available rebates and rising energy cost the calculated savings might pay for the new system in just a couple of years. One thing is certain, energy cost will continue to rise. With fuel surcharges and now demand based rates, that charge you more for electricity right when you need it most, an energy efficient air conditioning system is not a luxury, but a necessity! Air conditioning accounts for 50% of your homes electric bill on average.