In the beginning, they can be a bit wonky until you get used to best place to put them on your body. If, for example, you put them somewhere the sensor and transmitter gets buried into your mattress, your readings can drop up to 50 points until it regains line of sight. If you bury sensor/transmitter spooning a partner, you will get complete loss of signal. These are just the little nuisances you need to overcome and once you do, the sensors are usually stable. If you hit a blood vessel putting in a sensor, you may also get false readings.
Replacements come out of California or Mississippi 3 day FedEx depending on your location.
First month on Dexcom is usually a learning curve and a lot of OMG moments when you realize how certain foods send your BG all over the place.
Second Month is usually, " I always wondered how I would react to this food or this situation" and BG goes a little all over the place.
Third month you start really managing your BG starting with determining and implementing best time to pre-bolus so that you flat-line after eating rather than having a high BG 1 hour after food which is then hard to bring down, You will have learned that keeping BG low is much easier than trying to Bring BG back down without a BG roller coaster ride.
Lots to learn, but you will be amazed at how much easier it will be to control your diabetes and bring down your A1C