Sorry to hear you're having issues :| I've been in the "mine works awesome" camp since starting it. Yes, the sensors definitely need some help to stay on, but my readings (for the most part) have been extremely accurate. The things I have learned to accept with my G4 are that it's a great device to keep track of trends and how food/exercise/life is effecting me. It is not a full replacement for finger sticks. At first I was worried that the G4 would save 99 and a finger stick would be at 124. However, with the standard deviation of results, I can get see the same thing using 2 finger sticks. Yes, there are times when I look at my G4 and it says 110 and I stick test and it's 110. But a 20 point difference is quite common.
Also, as someone else said. Do not calibrate while you have anything other than a ---> arrow. I've noticed that readings at that time are about 10 minutes off of real time. So a 90 with a down arrorow usually results in a FS test of +/- 80.
Just to add a couple more points. I've been a CGM user for about 7 years, first with the Freestyle Navigator, then the 7+ and now the G4. I put a G4 sensor on my upper arm on Sunday and had terrible luck; lousy accuracy and lots of ???. Finally I removed it last night and found the sensor wire quite bent up. This always causes problems for the Dex so I kind of expected it. I think the Dexcom sensor wire, and the G4 more than the 7+, is very prone to bending when hitting any muscle tissue at all, which is likely what happend on my upper arm. I moved the sensor down lower on the same arm and more toward the back last night and it's been great since. Also, I use Skin Tac to improve adhesion (which works great) and I've noticed that if you get ANY of this on the location where the sensor wire inserts it will dramatically affect accuracy and life.
As a side note, my Navigator absolutley never had these issues and I think it's because they use a 90-degree sensor wire that inserts only about 4-6mm. I could put that anywhere on my upper arm with no problems and accuracy was always spot on. Shame it's not on the market in the US anymore but rumor is the Nav2 is out in Europe so who knows.