There are several problems: updates aren't really continuous, they're based on readings from interstitial fluid rather than blood, etc. but as far as I'm concerned the biggest problem is that they require an implant, a device that sits just under the skin and measures glucose levels in fluid.
I really want this sort of functionality in a convergent device; can you imagine your cell phone telling you your overall health level? Stress level? Fatigue level? Now if there were just a way to measure the contents of body fluids without needles or implants.