A lot better than chemical rockets...
Much lower thrust than chemical, but runs much longer.
Chemical rockets can be ion rockets as well. You just have to IONIZE the exhaust. Hydrazine is an example.
You take the exhaust, exite the atoms in the exhaust (thus making ions), the ions have electrons move to a higher orbital level. Now your exhaust pressure goes up, thus increasing the thrust, with the same burn rate, and the same fuel mass.
As the exhaust leaves the magnetic field, the excited electons drop back into their normal orbital shell, releasing a photon (conservation of energy and all that); creating a pretty blue hue. Simple, huh?
Well, except that the thrust is tiny. Don't look for ion engines to lift a vehicle off the surface of the earth into outer space.
Depends on the application. If your mission requires a lot of delta-V right away, an ion thruster ain't gonna cut it. To get newton-level thrust from an ion thruster takes KILOWATTS of electrical power.