I haven't heard this speaker but can offer up my $.02 on the configuration.
In my opinion this is the right way to do MTM (or in this case WHW). You see, the vertical stack of woofers will cause a radiation pattern narrowing in the vertical plane, and the horn has a radiation pattern that's wider than it is tall, so the matchup is a good one in that respect. You get symmetry in both horizontal and vertical planes, with the trade-off being a pretty tall box to get the tweeter up close to ear level. But the bigger box means deeper bass, so if the box size works for you that's a very viable approach.
This format will give a very high direct-to-reverberant sound ratio. This may or may not be desirable - there are divergent opinions on this subject. In general, a high direct-to-reverberant ratio is conducive to good clarity and imaging, at the possible expense of timbral richness and a sense of spaciousness. In my opinion you'd want minimal absorption in the room, but plenty of diffusion, so that the reverberant field is diffuse and decays fairly slowly.
The horn probably doesn't go much south of 1500 Hz, and running a 15" woofer up that high calls for some skill on the part of the designer. I would probably put a gentle dip in the crossover region to downplay any irregularities.
Overall this looks to me like a very promising design, and it's coming from a designer with a proven track record. I've built prototypes with the same general configuration (though with different drivers and horn), and I think it's a good one.
Duke