It's certainly frustrating. What's also annoying is the difference between sensors. Most of mine have been pretty good with hypos, as long as they were at the right velocity, in other words, by the time you felt it the scan was showing it or almost showing it. When you have a fast hypo, it is useless for checking as the twenty minutes delay can show an okay blood sugar (sometimes with a flat arrow indicating slow change) and you really can't see it as mentioned by Emmotha and smidge. For hypos, these things are pretty close to useless and you need to rely on your hypos symptoms and blood testing only.
In my opinion, the real use for these things is to understand the way your bg levels change in response to stimuli. I've said that from early on using it and my opinion hasn't changed. If you want accurate point values it's not worth getting the device and paying the money. If you need to understand how you respond to all your inputs it's very helpful, even when the sensor is reading 2mmol away from where it should be.
The problem is that you come to rely on it as being accurate when it is delayed data and too late to react to. For this reason alone I think care needs to be taken in its use.
