NC is good as is I think. Perhaps it's
slightly too powerfull.
IMO the early wonders are powerfull alternatives as well. It's especially the national wonders that don't compete. They need to be upgraded.
NC is by far the best National wonder because it's effect is powerfull, it comes early, and science is very important, especially then.
Ironworks comes second, the hammers are nice but the boost is not as powerfull and it also comes later. Unless you're OCCing you probably will have more than one city by now so it takes longer to get the requirement and production cost is also higher.
The National treasury is okay since you will probably build markets anyway (I do) but ten gold isn't huge so doesn't compete.
The National Epic is pretty weak with only 25% boost. Only really worth if when focussing on GP.
The Heroic Epic is certainly powerfull and doesn't come too late. Ofcourse only usefull for millitary strategies whereas NC is good for almost any strategy.
Hermitage comes a bit late, is therefor harder to get. I find I only build it when OCCing or going for a culture victory.
Circus Maximus is awesome for large empires. Ironicly, it's also hardest to get (as any NW) for large empires...
Oxford University. Free tech is always nice. But this is basicly a Great Library and never felt like an authentic national wonder to me. You'll need universities everywhere and it comes late. So still mediocre.
In fact, if we compare National Wonders. NC is by far the best with only Ironworks competing seriously. The rest is either weaker, later or more situational.
Edit: I created a thead to discuss the balance of National Wonders compared to the National College here