Simple problem: Want motion detection (occupancy) at both ends of a hallway or stairwell.
Solution: an amazingly large steaming hot mess!
[Update]: OK, so I've realized that the main reason why there are relatively few residential application for motion switches is: they don't save you money.
You can read the details here. The original reviewer makes the mistake of confusing a 40W LED bulb (320 mA draw) with a 40W equivalent LED bulb (actually 9W, 72 mA draw), but otherwise the concept is sound.
The reality is that unless you are switching large lighting loads (150W-ish or more) and/or in a commercial environment where lights might get left on 24/7, adding a motion sensor makes no sense. The "smart, energy-saving switches" will consume MORE energy than if you accidentally left the bathroom lights on an extra 15-20 hours per week, which seems rather unlikely for most situations.
This also means there is no payback period for smart switches. They end up costing you when you buy them, and costing more every year in the energy they consume.
There are still tons of reasons to buy motion sensors for the home. There is no need to struggle carrying loads, or tripping down dark stairs. Those things may easily be worth $5-$10 per year in energy costs, especially when compared to medical costs owing to an accident. It's cheap insurance. Ditto dimmers, which are just as much for comfort rather than energy saving.
But if you really want to save energy - after upgrading to LED bulbs, of course - the best way is to get up and turn off those LED lights whenever convenient, using a good old-fashioned $2 mechanical switch. Everything will be off - totally off - and consuming zero energy, rather than consuming small amounts continuously to be "smart".
This rather puts California's Title 24 in doubt, at least when it comes to residential applications. But whatever.
As one reviewer commented, I have difficulty visualizing a situation where you don't want two motion sensors (at the end of a hallway, opposite sides of an L-shaped room, etc.) Yet the solution is surprisingly hard to find.
Note: "Occupancy" means lights turn ON and OFF automatically. "Vacancy" means lights turn OFF automatically, but have to be manually turned on first.
Lutron: Lutron advertises various "multi-location" Maestro Sensor switches. Most models start with "MS", and most do not need a neutral wire. Awesome, right?
The problem here is that you can only install ONE motion sensor, and the rest have to be "ordinary" 3-way switches. Not what we want.
One enterprising soul did manage to make two Lutron MS motion switches work together in a "3-way" application, but it does not function 100% normally. In a normal 3-way, ONE switch must be off to turn off the lights - in his setup, BOTH switches must be off to turn off the lights.
This is not a big deal (perhaps) since both lights are automatic, but may not suit everyone. If it does, this may be the only no-neutral-wire installation that is possible.
You also must have a ground wire, and the switch is reportedly relatively loud.
Leviton: Use the IPS15-1LZ (switch) and IPV0R-1LZ (remote sensor) together. The remote sensor works with the main motion sensing switch to do true 3-way motion sensing.
The problem here is that both of these units need a neutral wire in the wall box in order to function. Which not everyone will have.
It may be possible to substitute the green / bare ground wire for the required white neutral wire. I don't recommend this because the ground wire is not supposed to be used in this manner, but it will likely work. (The Maestro series actually does this on purpose.)
The other available Leviton switches seem to require one of the switches to be a "regular" 3-way mechanical switch.
Enerlites: The DWOS-3R is the only multi-location switch. It does not seem to be rated for use with LEDs, and it is unclear if you can actually use two switches controlling the same load.
Eaton: The OS310U-W-K is not rated for LEDs, and there is no indication you can use two.
Hubbell: According to the instructions, you can use two ATP2000 series or two WS2000 series motion-sensing wall switches in tandem, and they will work properly.
Of course, they have high cost, limited availability, and non-standard wiring. Other than that, they're perfect!
There are neutral-wire and no-neutral-required versions, and versions with and without a built-in nightlight. Assuming no nightlight and no neutral, the model number is ATP2000W or WS2000W.
For reference, the "ATP" switches have some kind of auto-adjustment technology that lets them adjust their timeout period. The "WS" series have an "ordinary", manually-set timeout period. Both types are 3-way capable.
These switches are rated for "most" LED and CFL bulbs, and it does use a triac instead of a relay, so YMMV.
It is also not clear if the wiring is one-to-one with existing 3-way wiring. Ordinary 3-way has line-hot to one switch, plus two travelers, while the Hubbel diagram shows line-hot going to both switches with one traveler. You could re-purpose a traveler to a hot, if you know how.
This has already taken an amazing amount of time for what it is - doing motion in a stairwell - and I haven't even looked at my existing wiring yet. I will update this if I learn anything more.