Recs.
Updated
Shows important statistics when building crafts (mass, deltaV, thrust, ISP - while on Kerbin, in vacuum, or on other atmospheric bodies) and in-flight (all of the above + periapsis, apoapsis, orbital period, time to ap, time to pe and many many more). Helps with design, flying, transfers, orbital encounters etc.
Specs
Pros
Pro Shows important statistics when building crafts and in-flight
Shows important statistics when building crafts (mass, deltaV, thrust, ISP - while on Kerbin, in vacuum, or on other atmospheric bodies) and in-flight (all of the above + periapsis, apoapsis, orbital period, time to ap, time to pe and many many more).
Pro This is not an autopilot
As opposed to MechJeb, which is considered "cheats". KER helps a ton by providing already available info in a very accessible manner.
Cons
Con Less readable than some alternatives
The same information is presented in a more intuitive and readable way by DMagic's "Basic Delta V" and "Basic Orbit".