I've had good luck with the D3 so far -- the only exception was running Widget Lockscreen for a day or two, and the phone heated up and the battery drained in a few hours (a thread on that suggests the dev has fixed the problem). As a result, I moved from the free to the paid version of System Panel, which helps with tracking resource use over a week.
But more to the point, I want to second what BayouFlyFisher noted: in particular, having owned the Viewsonic gTab -- and having rooted and reinstalled ROMs on it dozens of times -- I can attest that there are all kinds of subtle ways that apps can misbehave. Those rogue apps that are poorly coded. Stale cache data in the Dalvik cache. Odd interactions between apps trying to use the same resources.
At the moment we own 2 D3's and typically come home each day with 70%-80% battery. Admittedly we don't use them all day, but even with occasional email and web I don't think we've ever finished the day near 50%. I'm not sure if that's typical or freaky for the D3, but the one time I saw a rapid drop (Widget Lockscreen) it was obvious that something was using resources too fast.
If the cause of the problem isn't obvious, what I learned from wiping the gTab over and over is that a factory reset and starting as clean as possible is a good strategy. Definitely the best approach to figure out if it's software or hardware, and if software, who the culprit is!
-Matt
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