Like many things the durability of the system depends on the build quality, the installation standard, and of course how well you maintain it. However, I've allways been informed that geothermal is durable and long lasting. Aparently the compressor has a life expectancy of 20+ years, and the wells or coolant pipes are expected to outlast the exchanger by some decades. I find these figures plausable as a number of very old geothermal systems are still in use, some are 30 or even 50 years old now, although I'm sure they have been repaired a few times by now. I suspect the good life span is due to their having few moving parts, and that all the machinery is inside the house protected from the weather.

Repairing the compressor unit isn't going to be more expencive than any other furnace from what I've heard, but there's a small chance of a coolant loop leak, and those can be expencive. Fortunatly they're unusual and generally happen soon after installation as poor installation is the most common cause, so the warranty should cover these cases. Otherwise it's a small risk you will have to accept. However, other heating systems can also have expencive or unplesant failures such as a dammaged chimney that can cost thousands to repair, a burst water pipe that can wreck your house, or even in the case of fuel burning appliances they could potentially set fire to your house. There are risks to all systems, some worse than others, but all can be minimised with a good installation. It's unfair to mention the worst case example when it's rare, and particuarly unfair as the worst case example of other systems can be just as bad if not worse.