[Kuruharan]

I'm going to make a definitive statement from right here in this comfortable position of straddling the fence.

I'd say, both better and worse. Whether the good outweighed the bad or vice versa would depend on the perspective.

Feanor did accomplish some things that were good in and of themselves.

The adaptation of the Tengwar, the creation of the Silmarils, and the palantiri, to name just a few.

However, he also did some things that were just downright naughty.

The Oath of Feanor and the Rebellion of the Noldor, the Kinslaying, abandoning most of the Noldor in crossing the Sea, (and some sarcastic souls might say that his having children was an unmitigated catastrophe).

An interesting thing is that if one examines the aftermath (or wreckage) of Feanor's actions, several things ultimately had an impact quite opposite from the original intent.

The creation of the Silmarils, which was not an act evil of itself, resulted in all the disasters that overtook the Noldor later. And on a much more minor scale (which is almost pitiful to note) the palantiri were turned to evil by Sauron and resulted in the overthrow of the minds of Saurman and Denethor.

But on the other hand, the Revolt of the Noldor brought the High Elves back to Middle-Earth which resulted in the edification and enlightenment of humanity, who were ultimately destined to replace the Elves in Middle-Earth. This would not have happened to the same degree if the Noldor had not returned. (No disrespect to the Sindar, but...)

So, in the end, I believe that the exact opposite of Niggle's statement at the end of Tree and Leaf applies to Feanor, it is hard to say if things in Arda would have been better or worse without him, but they would certainly have been different.