I'm late to replying to this thread, but I'd like to offer a few words that I hope may help.
I've been on the Dexcom G5 since last September when it came out and I LITERALLY trust my life to it....but I had to learn how to use it first. I think the number one thing I've learned in that time what that there is a time to blame the CGM, a time to blame technology in general, and a time to blame it on user error. I'd dare to say that the CGM is to blame less than 5% of the time.
Common frustrations:
1.) The readings are off- off from what? A standard glucometer has a margin of error also. Plus, they take measurements in two different ways as mentioned previously. It may the the CGM's fault or you could be dehydrated, calibrated it incorrectly, restricting blood flow to that area (especially if you sleep on it), or similar issues.
2.) Signal loss- while this happens much less frequently with the G4 (vs the g5) it still happens. I know it's particularly frustrating when it's a medical device, but that doesn't mean it's except from signal interference just like any other device. If you're ever curious, there are Bluetooth signal detector apps in the Apple App Store that will detect all of the Bluetooth signals around you. After I realized that my CGM was trying to stay connected to a signal device in a sea of more than 30 devices (airports are nightmares) I appreciated it's consistency much more.
A bit about what works well for me:
I almost always calibrate the CGM when my blood sugar is most stable. That ensures the least margin of error rather than worrying about the 10-20 minute delay in readings.
Occasionally, I will calibrate on the peak of a high or a low, but that's a much more complicated process that took time to learn as you have to consider what the trend arrows are telling you. Example: if my CGM says I'm at 9 mmol/l and the trend arrow says I'm still climbing then I wouldn't calibrate because it could just be the delay. However, if it says I'm at 9 and the arrow (and graph) shows I've reached a peak, but my glucometer says I'm really at 12mmol/l then I may calibrate the CGM.
What I deal with/dealt with:
I often get false hypo alerts (my family members get the alerts too) in the middle of the night because I roll over on my side and sleep on the sensor/transmitter. I've shared the full story on here before, but it happened once where my family called me and I didn't hear my phone (ring 25 times). I eventually woke up to paramedics at my door even though my blood sugar was really around 5-6 and perfectly okay. In the end, this was my fault not the CGM.
I used to get horrible sensor loss issues. It was particularly frustrating because it seemed to happen right after I went to sleep which rendered the CGM useless at night. I eventually realized that it was an issue with the iPhone software and running too many background apps. Again, not the CGMs fault.
I've wasted 2 sensors trying to install them on the back of my arms like some people do. It just doesn't work for me. Not the CGMs fault. However, every once in a while (maybe 1 in 15) I get a sensor that doesn't work even when installed perfectly. Dexcom has always been great about replacing them for me. It's a pain and I do blame it on the CGM, but I know Dexcom is working on a new sensor with a single push button applicator.
Just a few thoughts. Hopefully there's something that may be valuable in all of that