I've owned a number of Magneplanars in the past, but my first real exposure to open-baffle was when I built a set of GR-Research NX-Oticas with OB servo subs a few years ago. This has been the most enjoyable system I've owned. I recently replaced the NX-Oticas with GR-Research Line Force speakers, which I would also consider open-baffle, but they use a line array of B&G planar-magnetic drivers.
I have since heard a couple pair of Linkwitz speakers and the Spatial Audio X4s.
All these open baffles have a characteristic sound - somewhat similar to panel speakers, but with improved dynamics (I haven't heard any Magneplanars newer than the 3.5r I owned, or their top models).
Compared to the various box speakers I've owned (Duntech Sovereigns, Revel Gems, Revel Studios, Revel F50s, Totem Mani-2s, Goldenear Triton 2+), the open baffle speakers presented a deeper, wider sound stage where the speakers completely disappear, but the imaging is not quite as sharp.
The open-baffle bass does not pressurize the room as much, but is still very powerful with excellent definition. I find this a big advantage for music, but probably wouldn't work as well for movie sound effects.
At this point, I can't see myself ever going back to box speakers. When I owned maggies, I always felt something was missing (although I enjoyed their sound very much), but this has not been the case with the NX-Oticas and OB subs.
I've only had the Line Forces running for a week. Compared to the NX-Oticas, the sound stage is a bit wider, the presentation (particularly vocals) is a bit more forward, and they have considerably more detail and sharper transients. They are quite a bit more efficient (at around 98db/w), so part of the improvements I'm hearing could be that they are a better match for my 300B Parallel SET amp. But even with limited break-in, these speakers are pretty stunning.
I think all speakers sound best when they are given some space in the room (e.g. away from room boundaries), but open baffle speakers really demand this to achieve their magic. I have mine 7 feet from the front wall and about 3 1/2 feet from the side walls.