Xioami is a very big consumer products company in China. Aqara is one of their lines.
Pros:
Very inexpensive
Nice looking
Decent battery life for an inexpensive line
Cons:
Although they are certified for the same zigbee protocols that smartthings uses, they really don’t intend their devices to be used with anything except their own gateway, and they are, as engineers will put it, “idiosyncratic,” Particularly in the timing of their messages.
All of which means that they can be really fiddly to get paired, They can have trouble staying paired, and they only work well with some zigbee repeaters.
So the trade off for the money you save is that you may have to spend a lot of extra time troubleshooting.
You also have to use custom code with them and on occasion there’s been a hub update which then caused all Aqara devices to fail for a while.
For many people, very low-cost makes that amount of trouble worth it. That’s a personal decision.
Speaking just for myself, I might use some with their own gateway, but I wouldn’t use them with smartthings. But that’s just me. 
As @MinerJason mentioned the Ikea Tradfri bulbs and pocket sockets Happen to work well with both smartthings and Aqara, so those are good repeaters to have.
You can read more about them and find the custom code in the following thread:
[OBSOLETE] Original & Aqara Xiaomi Zigbee Sensors (contact, temp, motion, button, outlet, leak, etc)