It's been a few years since Neverwinter Nights 2 came out - if the problem with that game was that they were an "amateur" game company compared to Bioware, then perhaps that's changed in the interim; and they've acquired more experience by now. Obsidian's got a deal of history of making sequels and spinoffs from other games, after all.
My only real experience with Obsidian itself, so far, has been with KOTOR 2. And I was largely pleased with that game. It had a couple bugs, but I didn't encounter any more than in the first one - or pretty much any other RPG I've played. And there were a couple of things I didn't like as well (like how the influence system with your party worked out,) but I also can't really blame them for trying something a little bit different that just didn't quite work out as well as I'd hoped, either. I mean, they got points for trying, in my book.
I think probably a more reliable metric is going to be to see how Alpha Protocol turns out. I'm curious to see what they do with that, first. Plus I'm going with the giving them the benefit of the doubt since they've got some people on their team who actually worked on the original Fallout games. I don't see how they can do anything to the franchise that Bethesda already hasn't, personally. (And I actually like Fallout 3 - but I do find it ironic that there's people who are worried about what the remnants of the "original" team of designers are going to do to a franchise, when the last already was a rather large departure from what had gone before...)
Edit: In other words, if I have to be mature about Fallout 3, and remember that regardless of what downfalls a game has, it doesn't affect the "purity" of the other games I hold up as a standard for quality - then I expect the same from everyone else in regards to everything that comes out after Fallout 3. Even if New Vegas turns out to be the "worst" game since Brotherhood of Steel (which I doubt will happen,) the only thing it's going to ruin is itself. Fallout 3 (and the continuation of the series) will remain what it is regardless of what comes after.
It's like the X-Com series. Nothing good ever came of trying to do anything different with the series after X-Com Apocalypse. They weren't what fans were looking for, and weren't even good enough to stand on their own merits as decent games. But none of the flopped spin-offs did anytihng to detract from the quality of the 3 very good games in the series - those are still what they always were. They didn't "ruin" the series - they just didn't add anything good to it.
This is only going to be a win/win for Bethesda itself. If the game is overall highly-regarded, then they still get to take a lot of the credit for it (and I don't know any of the business aspects of it, but probably some of the money, as well.) They might even learn a thing or two by seeing their formula interpreted by another team, which would lead to a better all-round Fallout 4, eventually. If it gets massively panned, then they get to just forget about it and move on - and would probably get everyone hyped even more about Fallout 4 (to see the "real" developers do it right.

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