I enjoy not having to attach a magnet to a spoke to use this sensor. But speed gets erratic when cycling over bridges and roads with a lot of steel in them. The sensor seems to get confused or sends distorted signals to my Garmin head unit. Some others in our club notice the same thing.
Provides a more accurate measure of distance and speed if satellite reception is weak, like in an area with lots of tunnels. However, you should do an manual wheel roll-out with a metric tape measure. The sensor basically counts wheel rotations, just like the old wired cyclometers, so it's only as accurate as your wheel roll-out. The Garmin devices will attempt to estimate your wheel diameter for you, but it is wildly inaccurate. Don't trust it.
Garmin GPS head units typically under report distance when riding winding trails with switchbacks. Adding a Garmin Speed Sensor to your bike fixes this issue and provides real distance recorded at the wheel. Highly recommended.