I was looking for an ANT+ speed and cadence sensor for my Elephant NFE and went for the Wahoo Fitness Speed and Cadence sensor, which seems to be one of the cheapest units (5900 yen on Amazon JP) offering this functionality.
For comparison, the widely used Garmin GSC-10 is 19,700 yen on Amazon. The newer magnetless units released by Garmin in 2014 are available for 6480 yen for just the speed sensor or 9450 yen for both speed and cadence.
Why a speed sensor if I already have a GPS? For one, without the speed sensor my GPS has no way of measuring distance inside tunnels where there is no satellite reception. This has several drawbacks. During a brevet ride on a route with sufficient tunnels, the indicated distance from the start will drift further and further from distances indicated on the cue sheet that tell me where the next check point is to be found or where I need to make turns.
If I upload the ride to Strava and RideWithGPS, Strava will fill in the missing distances, effectively inserting straight line distances for each tunnel while RWGPS seems to do no such thing. As a result, my Strava and RWGPS stats will significantly diverge. I never know how long my ride really was until I have uploaded it to Strava.
Several other metrics, such as the gradient in percent or the climb rate (VAM = velocità ascensionale media) are not very precise without accurate data for speed, i.e. from a speed sensor.
On my Bike Friday I use a separate speed sensor and cadence sensor by Bontrager. They basically look the same and work the same, except one identifies itself as the speed sensor and the other as a cadence sensor. This is useful with the small wheels of the Bike Friday as the turning circle of the cranks and the rear wheel don't overlap, therefore a combo unit like the Garmin GSC-10 won't work. With the Bontrager solution you have total flexibility: I have the speed sensor on the front wheel and the cadence sensor on the non-drive side chain stay. Based on my positive experiences I would have bought another pair of Bontrager sensors if I could still have found them available, but no such luck.
The Wahoo solution is sort of a halfway house between the combo unit of the GSC-10 and the independent units by Bontrager. The speed and cadence sensors are connected by a short piece of cable, which allows you to position the two units somewhat independently of each other. Some people have reportedly even cut the cable and soldered in an extension cable (voiding the warranty, of course) for non-standard setups like recumbents.
When I first tried to install the Wahoo sensors the day before my 300 km brevet, I followed the instructions on the enclosed sheet. I was told to install the speed sensor on top of the non-drive side chain stay. I did not like the looks of that: If something ever pushes the sensor (for example, when you lean the bike against something during a stop or if your heels happen to hit it) that pushes the sensor directly into the path of the spokes of the rear wheel, which will slam it from the top. If mounted below the chain stay, the spokes will push it back out of the way instead of doing damage.
Initially I could make the speed sensor work but had trouble getting the GPS to pick up cadence data. Finally I ran out of time and took the Wahoo off the bike again before heading to the start.
Today I took another stab at it, using a different set of instructions (PDF here) from Wahoo, which recommended placing the speed sensor below the chain stay. That seems much safer. Closely following the new instructions, I had no problems getting the cadence sensor to work either.
The pedal magnet is attached to the left crank arm using a single zip tie while the spoke magnet is clamped on a spoke. The two sensor units are attached to the chain stay with a pair of zip ties each. Alignment was fairly straightforward.
My o_synce navi2coach GPS paired with the combined sensor without problems and as soon as I started recording, it showed numbers for speed and cadence when I spun the rear wheel by turning the pedals by hand.
While the Bontrager by necessity uses separate CR 2032 cells to power both units, a single cell will serve both with the cable-connected Wahoo. This is the same type of button cell also used by the Garmin GSC-10 and the Garmin heart rate belt. You can buy packs of ten very cheaply on Amazon and each should last for months.
So far the unit seems to work as advertised. I will have a chance to fully try it when I do my next century ride later this month (which will extend my record to 45 months of "one century a month").