Is Left 4 Dead's single-player mode tedious?

In short: it quickly becomes tedious.

The AI is only suitable for Easy or Normal single player. Beating Advanced and Expert usually comes from team tactics as opposed to FPS skill, and tactics requires human cooperation. You'll find yourself constantly losing if you're using AI on those difficulties.

The AI has some other shortcomings. They are incapable of throwing grenades, meaning your team's stock of weapons is less than that of a human team. Also, they will heal you if you're hurt, no matter the situation. If this happens to you when it's crucial that you run as opposed to stop and heal, then keep a medkit on you and pull it out to prevent the AI from attempting to heal you.

AI behavior is correct, but sometimes they aren't fast enough to react... it totally depends on the situation.

You can either win or die, without being able to understand why they didn't help you...

The harder the game mode, the more the AI will fail... =)

Left-4-Dead has pretty decent A.I. Better than most games. Generally, they'll stay with you, and provide decent support. They rarely get stuck somewhere in the level, and will run away when you run away. They will also almost always come to your aid if you need it.

That being said, they certainly aren't perfect, but nonetheless, I find that they're decent enough to warrant playing the game single-player.