The Edge 305 with barometric and GPS has very accurate elevation data when run through Garmin Training Center (the standard software that comes with the unit). Running it through something like Motion Based will give you an artificial increase in your elevation data.

I have only had one weird elevation anomaly with the Edge - and 7 of 10 people I was riding with that day had Edges and all reported this weird anomaly. We were climbing and the units showed a steady descent. We figure we must have been in some sort of satellite black hole. Overall, the data given after the ride was accurate (the mileages and GTC climbing data were consistent with prior Polar and Ciclo readings for this ride - just this one stretch of road (with no tree cover even) where the readings were wonky. That's the only time I have seen something funky with the elevation for the edge.