Horns provide a very forward, immediate sound, with lots of presence. Too much presence, having a severe "cupped-hands around a mouth"/"pinched nose" coloration that quickly becomes irksome. The worst speakers, by far, I've ever heard for vocals. The ones I've heard, anyway. But very dynamic, if that's your priority and you can live with the extreme level of coloration they possess. They do mimic the sound of the PA systems at rock concerts, which may be why rockers don't mind them. When I lived in my Band's house at the age of 21, our PA (Altec Voice Of The Theatre A-7's) did double duty as our stereo speakers.
Horn speakers (or horn hybrids, as they mostly are) span many varieties, and I don't find above descriptions to be a common denominator on their better implementations. With my own horn speakers (based on the Belle Klipsch) as a point of reference: rather than sounding forward per se I agree they provide presence, immediacy and dynamics in spades (undeniable qualities, to me), but it's worth noticing how their sound morphs depending on the recording; some, if not most of the more modern rock/pop recordings can sound flat, uninvolving and compressed (which they are), but put on (particularly older) jazz titles and classical music and the soundscape suddenly blooms into a vivid, colorful and very dynamic presentation. I've never heard the likes of violin, cello, piano, clarinet, saxophone etc. sound that authentic, full of life and presence. Dynamics, both micro and macro, isn't just a priority here; it's an outright necessity if one is to emulate the imprinting of live instrumental sound, as is truth of tone, sheer air displacement area, ease and overall coherency. Perhaps much of the controversy of horn speakers resides in their unsuccessful pairings, be that both with the associated equipment as with the music (i.e.: their recordings). Add to that the inertia of what audiophiles are used to hearing from mostly (smaller) direct radiating speakers, and horn equivalents can be found to sound "wrong" simply by virtue of having a different, more direct presentation.