I've had good luck with my iGrill as well. I have noticed that direct sunlight can cause the sending unit to get hot and quit sending. To solve the problem I keep it in the shade. (I hammered a nail on the inner skirt of the side table I built where I hang the iGrill) I haven't had any issues since I started doing that.
I've also noticed the iPhone uses the battery up pretty quick when paired with the iGrill. I keep the iPhone plugged in while using the iGrill to solve that issue.
Here's a graph from a 15 hour cook with no iGrill issues: