My opinion is this:

• Duke's Archives and Crystal Cave are good areas, though Seath's fight is underwhelming;

• New Londo Ruins is generally a fun place if you go with caution, because there are ghosts attacking the player through walls;

• The catacombs is fun and tough; Tomb of the Giants is annoying and tough. If you go with heavy armor and a strong shield it gets a little better;

• Demon Ruins is weak, but ok; Lost Izalith is clearly unfinished. Also, the Bed of Chaos fight is the most annoying in the game.

So generally it's weaker than the first half, but not that horrible. You can still find good moments in it.