I have one of those too, Melissa. I'm firmly on the "waste of money" team just because what's right for one plant is wrong for the next as far as moisture goes.
You can also have a plant that's dried out too much, but when you water it the water just whizzes right through the pot and isn't absorbed. So your moisture meter would tell you the soil was bone dry even though you watered the plant 5 minutes ago. It takes a good long soak in a bucket to re-hydrate dried up potting soil.
I used it for a short time and found it really didn't tell me anything I needed to know, and sometimes told me wrong information. Your fingers will tell you more.
