Fun, but it can get old fast. PC version gets a little savior from the fact all the DLC is free, and you can download player-made maps as well. However on the 360 the DLC is not free, and the maps that come with the game don't go very far. But you should realize everything is orchestrated by an AI called The Director. One playthrough will not be the same as the next. The Director will "watch" the game and decide what enemies to place where, and when. If you go too fast it may decide to slow you down by spawning a Tank. If one person leaves everyone else and runs ahead, chances are it will spawn a Smoker.
That's not to say it isn't fun, it's fun as hell... but only with friends. You can get some solo enjoyment out of it, but to really get the most fun out of it you gotta play it with at least one other person(because of the way the game design works, you can't save yourself and AI can be ****ty at prioritizing). Then after that you can get into versus which can be really fun as well for a while.
My only hope for the new L4D(if there is one) is that they really change the way they do maps. In the first one maps can range from taking a couple of minutes to complete to about 15 minutes. They're predefined, and pretty linear. In L4D2 they claimed they maps had a dynamic layout in which The Director would change... but honestly I played the hell out of L4D2 and barely noticed anything different in level layouts, but they were a lot longer. Maybe once or twice I'd go down one alley instead of another, but that's it. In order to truly make the games get better replayability they need to design very large maps(like whole towns) and actually designate entirely different paths and layouts for the survivors to go through. Only then will each playthrough seem totally unique.