heart rate monitors

I recently bought a vivofit with a heart rate monitor, but I'm wondering what you all think about them. I'm not sure if it's all that accurate, the timing seems a bit off, I worked out for about 50 minutes and it's saying that I worked out for 72 minutes, also it automatically logs it as a walk, jog or running when I'm doing indoor cardio like zumba and 30 day shred. Just wondering if any of you have any insight to this or have any advice on how to adjust or fix the issue.