I've done accuracy studies of the G6 for a submission to the FDA, and while it's true they do filter out spikes, they're never more than 5mg/dL off from the measured value (which itself is usually within 10% of the real value). Interstitial lags are closer to 5 minutes when you're not experiencing severe perfusion issues, which is more than close enough for making dosing decision, especially considering subcutaneous insulin has similar or longer lag times (depending on lots of things of course, and subject to an individual's own body).