The singleplayer is about the same. Halo 4 has much better characters than Reach's bland soldier parade, and it's story is fortunately much less down to earth. Where in Reach's campaign everyone could have been swapped with generic soldiers, this game's vide is truly alien. The graphics are astonishing help convey this theme. However, there are not as much vehicle sections as in 3 (about as much as Reach) and the new enemies aren't as fun to fight as Covenant.
The multiplayer is a collosal step forward from Reach. There are a lot of small flaws, but overall 343 really NAILED the map design and weapon balance, making this one of the most enjoyable competitive multiplayers this generation.
I would give it an 8/10, compared to 3's and Reach's 9/10 and 6/10 respectively.