Its a couple of things.
The efficiency is really nice - it doesn't take much power to fill the room.
The sound is less cluttered and more direct than with the 3 or 4 ways systems -
there is an immediacy or presence to the sound, a clarity, as if there isn't much between me and the sound.
There is also a focus to the sound image - the soundstage is crisp and clear.
Its also nice that they are apparently not in the collector's eyes right now -
these days when anything with a JBL badge gets listed at a high price,
the UREIs have an outstanding value. (Hidden JBL parts!)