I think it is truly amazing. When I first bought L4D, and played it, it went over well, but not this well. This game was well worth the money and more. Though I'm not complaining about the game's price tag, after playing it, I think it's more deserving of the $40 price tag than L4D.
But, aside from comparing it, it is an excellent game. I regret not playing the mod to see what all's been improved, but I love the gun selection. No gun feels underpowered, and I love the whole only certain guns have flashlights thing. Gives you a reason to use your default pistols.
Every gun has a time and place where it is most effective. Right now I'm loving the handcannons, Hunting shotty, and the Lever action rifle.
I played West London, the bio lab, and the farm, and all three were majorly different experiences. In WL, you've got the streets crawling with these things, and seeing the place on fire, and abandoned, bloody, it's amazing. The lab, you've got this abandoned, industrial murder chamber basically, where the halls are small, and filled with the bastards. Very intense, dark too. Then the farm, light is rare, so you use your pistols a lot, and it's big, and open. Easy to get seperated from the team, not easy to get seperated from the specimens. Very, very atmospheric. Each map has its own feel. Many props to Tripwire for this.
This game gets a 10/10 from me. Amazing experience. I'm off to watch a tad of TV, eat, then it's back to KF.
EDIT: I forgot to mention, each round, the people I played with were nice. Nobody got voted to kick, and they didn't yell at people who don't have a mic. Nice people. I think this game's got a long future ahead of it.