For $15, absolutely.
I enjoyed the campaigns a lot more than the first one. They seemed to have a lot more depth and flow together much better - rather than "we're in a city surrounded by zombies" it felt more like "how can we get to such-and-such a place and figure out if the entire world is infected, or just this area?" I found the Survival mode areas much more enjoyable than L4D, though I do miss Last Stand (lighthouse). Versus is better in one way, and worse in another. Having more special infected to "choose from" (randomly assigned) is nice, but every now and then you wish you could just get the normal smoker, boomer, hunter(s) combo for a quick kill. There's nothing worse than getting a charger in a small room full of obstacles or a jockey in a wide open area with no cover.
Character wise, the original was much better. The voice acting seemed a bit better, and the characters generally seemed to get along and interact through dialog better than the new crew. I find that Coach and Nick are somewhat enjoyable, and Ellis isn't too bad. Rochelle is far more annoying that Zoe ever could have been, though. For a game that's not about NPC interactions, it's something I can overlook.
Having more weapons available (not just melee weapons) was a plus, though I wish they had less similarities to make the choice more difficult (for example, one sniper rifle could be extremely powerful, killing any special infected other than the tank in one hit, but take a very long time to reload). Melee weapons are so-so. Sometimes they feel overpowered, and sometimes they feel useless.
Overall, I wouldn't pay $40-50 for it, but I feel anywhere around $20-30ish is perfect. $15 is a steal.