If your blood glucose is steady, they're great at telling you small shifts (at no more than 4mmol/hr, I'd guess). They don't measure blood glucose, so if it's swinging rapidly the reading will seem "late" (about 20 mins late). Remember this. Any reading on a CGM is from 20 mins ago.
I used the Libre when it first came out. I realised how handy CGM is, because most of the time I really only want to know if my BG is rising or falling (& how fast). It's great for checking on overnight BG. But the Libre, it's a bit of a pain to keep waving the box over your arm. I tried putting the sensor under my arm - it didn't work. As these things are really expensive, it was a shame to see £50 burnt up.
I was fortunate to get a very well paid contract in Scotland, so one of my first luxuries was to buy into the Dexcom G4 (£1600 startup *SQUEAK*)
I can spin their sensor out from their recommended 7 days to just beyond 17 days (the "break-even" sensor-cost between Libre & Dexcom) so it costs me a fraction less to run than a Libre. BUT I find they do get a bit flaky beyond 2 weeks (the sensor stops giving reading for an hour or two). Beyond 3 weeks, they're losing the ability to see rapid changes (showing only a slow drift and never really getting to the "true reading" on your BG meter, oh and they're dropping out for 3-4hr chunks by then, but it seems to be related to orientation and movement - still researching this
)
So, in conclusion, CGM - Great! Expensive. Dexcom better than Libre, but it's REALLY expensive.
All in all, I wouldn't go back to Libre, the alarms (on lows & highs) are really useful when you're busy at work. The other downside is that it's ANOTHER box to carry around.