system
Given the following post, respond as if you're commenting on Reddit:
human
What are the advantages and downsides of an Ion Propulsion system over conventional spacecraft propulsion systems, like liquid fuel?
gpt
There are several things that are interesting with ion thrusters and electric propulsion systems for spacecraft in general. 

The most obvious one is that they can accelerate propellant to much higher speed. This means that the certain quantity of propellant will generate more impulse. Impulse is the thrust time the duration of thrust. 1 kg of propellant in a liquid fuel engine can push with 1 kg force for around 450s maximum, for a ion thruster you can push with 1 kg force for up to 5000 seconds. So it's pretty obvious that ion thruster need a lot less "fuel" to accelerate a spacecraft than liquid engines. In practice that means you can make the spacecraft lighter and either use a smaller and cheaper rocket or share a rocket with another spacecraft. 

However accelerating propellant to higher speed requires a lot more energy. Each time you double the speed you have to quadruple the energy input. Liquid rocket engines are already Incredible powerful systems producing megawatts to gigawatts of energy. For an ion thruster you have to get that energy somewhere and in space that means (for now) solar power. While it's pretty great because you don't run out of it, the power is pretty small. Big satellites today only generate about 20kW of energy which is about 25 hp. So combining low energy input and very energy hungry ion thrusters you get a super low thrust. The result is that you need to keep you engine on for weeks and months at a time to get anywhere. This is an issue because you spend a lot of time just going somewhere and not performing your mission. All this time wasted is time where the spacecraft has to endure radiation and has chances of breaking. It also doesn't make money when it's in transit. Moreover those slower transits are usually less efficient than big burst of thrust for short periods of time.

Those kind of long and complicated trajectories are also a lot harder to calculate and you need mission planners who really know what they are doing to work on those.

There are also system level considerations. Electric propulsion systems usually use non-toxic propellants which means it is less dangerous (and less expensive) to fill the reservoirs. This is somewhat negated by a lot of ion thrusters using expensive xenon gas. This is why a lot of people try to not use xenon anymore. 

Ion thrusters also don't necessarily make sense if you don't already have a payload that needs a lot of energy. Adding large solar panels just for a thruster doesn't always make economic sense.

As with all engineering problems this is a tradeoff you have to do carefully. That said nowadays electric propulsion systems represent the majority of propulsion systems installed on new spacecraft. People used to be very intimidated by the complexity and think ion thrusters were mostly expensive laboratory toys. Over the past few years we have seen a lot of companies doing "plug and play" systems that makes ion thrusters a lot more accessible.