We’re starting to see quite a few questions about MMwave sensors and there are now several that can be used with smartthings.
First, what they are
These are NOT “presence sensors.” Presence sensors tell you when a specific device has crossed a Geo boundary. (These are also sometimes called “arrival sensors.”) so “Michael just got home.” “Linda just left home.” They are typically WiFi, Bluetooth, or Zigbee. Your smart phone can act as a presence sensor. They are marked as arriving or leaving, because their signal can no longer be received by a device that they are supposed to check in with, or because they have GPS capability and can be located on a map.
In contrast, occupancy sensors don’t tell you who is in a room: they just tell you that some human is in the room. So they are also called “human presence sensors.” A person is in the room, but we don’t know whether it is Michael or Linda. Most of them can also recognize that there is more than one person in the room.
The ones that are available for home automation systems in 2023 almost all use “MMwave” technology, which is similar to radar on a very small scale.
Motion sensors, which use passive infrared technology, typically track changes in heat across the sensor lens. So they’re good at catching big movements like a person walking into a room. But they aren’t good at smaller movements, or even recognizing that a person is sitting on the couch.
MMWave sensors are great at fine movements and very fine movements like a person breathing. They capture a huge amount of data in a room. Many of them can divide a room into 20 or 30 zones and report separately on each zone.
Of course, since they are doing so much both in collecting data and evaluating it, at the time of this posting all the ones available for Home Automation systems need to be mains powered, not battery powered.
In commercial applications, these are popular for parking garages so they can tell exactly which spots are occupied. Some of them can go even further and tell if there is a person inside a car.
But for home use, they are typically used as an improvement over PIR motion sensors. They can tell when there are multiple people in the room. They can tell if someone is sleeping in the bed versus the room being empty. Stuff like that. Some of them can tell the difference between a person walking into the room and a person walking out of it, although I don’t think that feature is currently available with smartthings.
Also, MM wave sensors can detect through clear glass (but maybe not through fancy hi tech glass or UVB blocking glass or colored glass), which PIR sensors cannot. So another popular use case for these in Home Automation is to detect whether someone is in the shower stall in order to keep the bathroom lights on.
Costs have come way down in 2023. You can definitely find one for under $150, and you may be able to find one for around $85 if it’s on sale. Or even less if you look at the Tuya models. (Sonoff offers a single zone, non adjustable model for under $20, which is using mmWave technology but without the advanced intelligence of the other brands.)
The following is a good general discussion thread on the technology:
HOW DO THEY APPEAR IN SMARTTHINGS?
at the time of this posting, all of the available models just expose each zone as a motion sensor and then you can write routines based on there being motion or not in a zone. So one mmWave device might show up as 20 different motion sensors in one room. But to use a lot of the more advanced features, you generally have to use the Device’s own app, and that creates some integration challenges.
FEATURES
New ones are being introduced all the time. The ones that work with smartthings will be either zigbee or Wi-Fi. Zigbee ones will have to be either connected to their own hub and then brought into smartthings through a cloud to cloud integration, or connected to a smartthings hub (in which case you probably get fewer features or require custom code).
Some also have temperature and light sensors, but these are not always exposed to smartthings.
So you’re going to have a lot of research to do on individual models to see how and if they work with smartthings. Also, you need to check specific model numbers. For example, the aqara FP1 is a Zigbee Device, and the aqara FP2 is a Wi-Fi device.
Setup Takes Time
Setting up one of these is typically like creating the map for a robot vacuum or setting the activity zones for a security camera. Expect to take some time with the device’s own app to get everything set up the way you want. And it may take some trial and error to set the zones correctly. It can also be tricky if you have clear glass windows and you don’t want the sensor to detect activity outside. So getting everything set up the first time is a lot more work than it would be for a PIR motion sensor, but then these are much more complex devices. 
(Again the Sonoff is an exception: since it only has one zone and there are no settings you can change, there’s no special setup required.)
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My next post will list some specific models that currently work with smartthings. Expect to see more and more of these.
As to which is “best“ as with most sensors, there are a lot of different factors, including how much you want to spend, what specific features you need, whether you have a smartthings/Aeotec hub, etc.
If you are currently using one of these with smartthings, please feel free to add a review post to help others in the community. You can also link to your existing discussion threads, there are quite a few, although they are going out of date pretty quickly, because of all the new models being introduced. 
. Other way any one try the Tuya sensor ZY M10x? Works fine with any edge driver?