A lot of great observations and comments in this thread. I know it might seem that there are some fundamental differences in opinions expressed, but, I pretty much agree with everything said--differences being probably more a matter of degree or how one expresses what is observed, and matters of taste.
Charles1dad and Audiokinesis and others have mentioned the importance of good crossover design in both horn and direct radiator systems. I agree that this is a big issue. I think that is one of the strengths of horn-based systems when done right. Horn midrange drivers can cover a fairly wide band of frequencies so that the crossover points can be away from the 1,000 to 3,000 hz range where the ear is particularly sensitive to problems of speaker integration.
Isochronism, you asked about active crossovers. I have heard various horn and non-horn systems with active crossovers. I generally like active crossovers when properly implemented--the music sounds very clear, clean and more dynamic, even when high volume and additional power is NOT an issue. The "problem" is that those who have undertaken this approach have told me about how hard it actually is to get things right and the added complexity creates a lot of headaches (particularly when tube-based crossovers and amps are involved). The most impressive example of the difference between active and passive crossover that I heard involved the Active Advent speaker that the company put out; it was supposedly the same speaker as the conventional Large Advent, but with an active crossover and built in amplification. It in no respects sounded like an Advent to me--this thing had LIFE.
Audiokinesis, you mention "waveguide" horns. Do you mean systems where the horn is primarily acting as a means to control dispersion and where "loading" of the diaphragm with a column of air is not a primary part of the design? Also, the Classic Audio Reproduction speakers that you liked, were they the reproductions, like the Hartsfield, or the contemporary design like the T-3? I personally like both lines, perhaps more so the contemporary line.
Wharf, I have no idea if the Avantgarde Duo represents a "flawed" horn design with a lot of bandaids or if it is a correctly engineered horn design by whatever standards one could apply to make such a judgment. I do know that it is a hard system to setup (getting bass to balance correctly is extremely difficult). I also heard a certain "one note" quality to the bass even when it was properly set up. But, all those issues aside, that system can really deliver musical enjoyment. I also noticed that the Duo is quite amplifier friendly and sounded great with a fairly wide range of amplifiers I heard it with. You should, if you get the chance, hear it with an OTL amp--that is a lot of fun.