Although a little pricey for what it does these days, it is effective and far from an expensive investment.
The unit supports bluetooth, slowly but surely we are getting to the point where everyone will need BT just to be considered for purchase (that day can't arrive soon enough). The device has a memory and will work in concert to keep track of rides for uploads later. This can be hilarious and annoying when you are truing a wheel and the device thinks you went on 10 bike rides of two minutes each...
The argument for a speed sensing device becomes more difficult as phones are more ubiquitous and track via GPS as you ride (granted revolutions of a wheel will likely be more accurate than gps will approximate your path but should be negligible).
Since I'm a data nut, I find it hard to believe there will be a day I don't take this along with me. Originally I purchased the device for a second bike since I can never remember to swap these things.
Ultimately I would (and likely will) buy it again.
I now have three of these on my various bikes. I bought a Velocomp Aeropod which requires a speed sensor. Just got lazy moving the speed sensor to my different bikes. Works well when you go into tunnels and tree cover. Do not experience speed dropouts anymore.
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.
So after ten years or so of pretty hard use, the original speed/cadence sensor that came with my very first Garmin 510 finally gave up the ghost. I bought this one as a replacement. Mounting was a snap. Nothing align; just fasten an elastic band around the hub. It paired effortlessly with my Garmin 1030, and transmits data instantly and accurately. Really, it couldn't be any easier. Didn't need the cadence sensor anymore because I run a Stages power meter that includes a cadence sensor. Good price, fast shipping, easy installation, and flawless operation, even on the crappiest, wettest, coldest winter days. What's not to like?
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.