It's bad. Period. The game has that spawning shit that encourages you to rush forward into combat to trigger arbitrary checkpoints, AI that doesn't do anything, grenade spamming, useless friendly AI, shit level design, weapons that you can't really tell apart, repetitive campaign structure (night -> day level transitions are lazy)...it's just garbage. It's unfair.
The difficulty doesn't scale fairly. Halo's Legendary is easier than COD4 on normal, imo.
Multi removes all these problems and leaves you with satisfying gunplay and the interesting perks system. The single player is an abortion that looks especially bad after Half Life 2: Episode 2 and Halo 3.