I've been on the DexCom (7 Plus) since 2009, May. Long enough that pretty much all of the varying-experience scenarios reported here are inside my experience.
Doug, the only really important question I'd ask is, "Are you happier and better off managing the diabetes with the DexCom than you were without it?" If yes (...which is, by the way, my answer...), then even with its clear and (experience-obvious as opposed to not-quite-to-promises-)obvious shortcomings, it is well worth its expense and its incumbent frustrations. If not, then you're under no obligation to continue using it--ultimately (and I'm certain you know this, but it bears repeating...), you get to choose how you manage the disease.
For me, the DexCom has been a major godsend. I've seen the wide swings from finger-stick readings, but having the continuous data stream, and especially being able to see the trends is what I'm talking about. My A1c's are improved, and the incidence of BG crashes, while not zero (...although, it was zero for the first 18 months I used the CGM...) is way down from the three or four call-the-parameds incidents a year I was having before the CGM
Now where algorithms and such are concerned, somebody in the thread mentioned Kalman filtering. I have no direct view inside DexCom's engineering, but I know damn well they have to use some form of predictive number-crunching (which is precisely what Kalman filtering is an example of), since it's well known and understood that interstitial glucose levels lag (by 5 minutes...10 minutes...?) BG levels. There are also variables like where the sensor wire is inserted and whether or not that particular tissue is vascularized per DexCom design specs. (I have lipomas on my abdomen, and when I inadvertently stick a sensor into one of them, I usually have to toss it and start again within 24 hours--the infamous "???"s, doncha know...!)
Now in this context, I think the comments about the FDA are spot on. What they are really saying, without saying it (or, for that matter, permitting DexCom or anybody else to say it) is, "We don't trust you adolescent, pus-brained idiot, bone-headed members-of the-unwashed-masses, who have never been attorned into the brotherhood of Medical Validity Evaluators, to make any kind of judgment regarding your use of all these wide and varied things that you can only have access to if we say you can (...we, of course, being the only people on the face of the planet qualified to make such judgments on your, or anyone else's behalves...). So just shut up and trust us--that's what we're here for!" Frankly, I’d love to be able to write an app for my smart phone that acted as the DexCom receiver--one less gizmo to hang on my belt! But having no access to the design specs, or any of the engineering, or even what proprietary variation (if it's as I suspect) of BlueTooth protocol they're using, and since DexCom can't offer an Android app without getting massive FDA approval, any discussion of improving things on my own, or in the company of like-minded enthusiasts, is moot. Open source will never to happen with anything the FDA has hegemony over.
A sinecured bureaucrat, regardless his bona fides, is not, never has been, and never will be, a substitute for the synergy of an educated, informed, and motivated client ("patient ") working with a competent, informed, and interested professional consultant ("doctor" (...which, by the way, comes from the latin for "teacher"...)). All of the long, sordid history of bureaucratic interference under the aegis of "protecting the public" has not, in any demonstrable way I can see, ever managed to add to the "public welfare" (all the trumpet-voluntary protestations to the contrary notwithstanding), and in many demonstrable instances, has subtracted form it egregiously.
So maybe it's time to start a new thread about the efficacy and validity of government-managed health care--this thread being about DexCom accuracy, and all. Speaking once again from my own personal experience, I do find that the DexCom's accuracy, especially in the low ranges, is more than adequate to make my worries about BG crashes way less than they used to be, and to provide, overall, a significant improvement in my ability to manage the disease. I'm sticking with what works for me!