Can be good, can be bad, depends on how good an install you do. Most difficult part is getting midbass drivers that can blend in with them, and also keep up with them, horns are very efficient and it takes multiple midbass drivers to match them in output. As far as imaging, most horns give a strong center image with good focus, but they tend to lack side to side imaging. IMO, in comparison to conventional drivers, horns are easier to get to sound good, but more difficult to get them truly great.
horns have some advantages, like graet frequency range and imaging, but you pay dearly for that with difficulty in mounting due to size, the absolute need for 1.3 octave EQs to tune them so they sound decent in a car (absolutely needed, and always an expensive addition) and they have downfalls like group delay and a very distinctive sound that most people tire of very quickly after using horns in a car for a bit.
all in all you'd save money and get better results using a high end conventional set of separates in your car instead, like Rainbow Platinums for example, or DynAudio 240 Esotars. Both run about $1500-2000 US per set.. and both will yield a better overall result for transparency with close to the same output as horns with midbass drivers.