Left 4 Dead 2 is as good a game as I've seen, but has the most mixed reviews.
Nothing got worse, but alot of things got better.
For instance, L4D had very few weapons, and L4D2 has many new weapons and classes.
The new zombies are original, but not very useful expect in "what if" scenarios. Like the Jockey is useful IF the person is far, far away from the rest of the group, and there is a nearby hazard. Other wise, its failed jumps and a quick (re?)death. The same with the Charger and Hunter.
And, the Witch doesn't show up nearly enough outside the sugar mill.
The weapons and zombie variation is great, and the campaign maps are great. The zombie randomizer and no-reason horde attacks if you wait to long make it even better.