Just started with the G4 this past weekend, and wanted to give a brief review compared to the orginal Dexcom, in case anyone is on the fence about upgrading.
The executive summary is that so far, the G4 has been far better than the original, and the form factor/range is also much better.
Rcvr Size: Looking at the raw numbers when comparing the two is deceiving. In actual use, the G4 receiver is now thin enough to not be a bother when stored in a jeans pocket with my car keys. It seems to be less than half the volume of the original... The original was very large, leaving no space for anything else in the pocket, and the range was so short it had to be kept close to my body which combined to be a pretty big hassle. The new rcvr size is probably my favorite feature of the new model.
Range: There is no comparison. The new G4 works even when in another room, through multiple walls. The old one would occasionally lose signal if it was in the pants pocket on the opposite side of my body from the transmitter!
Transmitter: The transmitter is indeed a bit more bulky. I wear relatively loose shirts, and I never noticed the old tranmitter through the shirt. With the new one, I do sometimes see a bit of a wrinkle in the shirt, depending on how I'm standing/sitting. Not a big deal.
Comfort: About the same. Tape is the same. I might not be the best person to ask, as I never have issues with the stock tape, even when swimming or working out frequently.
Accuracy: The G4 is worlds better than the original, at least for me, for this first sensor. I had an issue with the original where it would run high for the first 3-4 days of a sensor, each time I calibrated I had to input a lower sugar reading. It was normal for it to be off by 30-50 points early in the cycle, if not more. Even at the end of the week, it would often be off by 20 points or so. When I asked about it here, the concensus was that perhaps my body was taking that long to reduce swelling/get used to the probe. On the other hand, with this first sensor on the G4, it has been accurate within 3-10 points every single time I have checked. I am only calibrating when asked (following the basic rules of not calibrating early in the morning or when my sugar is rising or dropping quickly), so this new accuracy doesn't seem to be due to lots of calibrations or any special usage. I don't know if this is just good luck on the first sensor, but I am thoroughly impressed.
UI: Honestly, I could care less about the color screen, but it is certainly nicer than the original. It does take longer to turn the screen on which is a bit irritating. I haven't used the new alert options, as I always use "vibrate". I do wish Dex would come up with a software update that allows different daytime and nighttime profiles, so I can expand my nightime alerts without so much button clicking.
Hope this helps anyone sitting on the fence. If you have any questions, just ask.