I have used a Vaaka Cadence for a number of years and recommend it -- never had any issues with it, had to replace the battery once (no big deal). Would get another if it died.
I use it with a Garmin Fenix 3 (using the Cycling activity), and there's an available Connect IQ data field by Vaaka for DPS (distance per stroke), that you can add to your screen. So in addition to seeing cadence, you can see real time DPS, which I find very useful for technique work.
