They work well for many people, but they are not standard devices and require special care in how you build your Zigbee network to keep the happy. Specifically, you have to use only known "compatible" repeaters or risk that your devices might choose one that doesn't work well.
This thread is good reading:
Personally, I got rid of almost all of mine but did use them with mostly good luck for a long time. The temperature/humidity sensor is all I use anymore because I haven't found another device I like quite as much, but I've also stopped caring about what repeaters I use and consider myself lucky they still work. 
