I had the original Palm Pre with WebOS. The OS itself was actually top notch. I absolutely loved it. I believe it failed because Palm just didn't have enough weight behind it (like Google or Apple have) to make it reach critical mass.

That said, this isn't the same as WebOS. WebOS ran javascript apps, but they ran on the device itself. From what I can gather, B2G is running everything on the actual web. It's very different.

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Edit: I may have been wrong there. Apparently it is running locally. This quote from the web page through me off:

"You don't have to pay the penalty of a big smart phone operating system if you're running directly on the web"

That's a bit misleading.