In general I think omnis including OHM are better at delivering a sound that seems more like a live performance despite being a recording. The large sweet spot and coherency of the soundstage from different listening positions is most indicative of this unique aspect and similar to what one would hear live. OHMs use of the wide range Walsh style driver to cover all frequencies up to 7khz or so is OHMs most unique feature compared to others. It delivers outstanding coherency to most of the music heard (vocals and midrange in particular) in comparison to most others with crossovers in this range. That along with the big and coherent sound stage from most any listening position is what makes the OHMS uniquely convincing IMHO.
Whereas more directional designs deliver more of the traditional two channel stereo recorded experience.
Having had my OHMs and more traditional alternatives set up for comparison in various rooms in my house, I always find the OHMs more convincing and involving musically overall even though the specific differences in the presentation are subtle and things more similar than different overall in most ways. Its just more lifelike!
I also agree that people either will take to the sound and never be satisified otherwise again or not. The difference can be subtle but significant! Others can come close but its not easy.
Other than that, in the end the other main advantage is the large sweet spot which either benefits peoples listening habits or not. If you are content being lodged into a small sweet spot to listen all the time, or coherency of soundstage from many locations does not matter to you, there are many more good choices out there than otherwise..