The only way to play l4d is Team Versus mode - you and 3 mates get all mic'd up and the (admittedly dodgy) matchmaking system pits you against another team. You can't join team versus lobbies at random, you have to either make the lobby yourself or be invited off the steam friends list, so you don't end up with anonymous cretins that spoil everything.
In a proper match, with both teams evenly skilled and voiping away, co-op is utterly essential, no way around it - running off by yourself will get you incapped/killed in moments; even splitting up into pairs is a surefire way to lose the round.
Single player/pubbing is mindless fun for a few rounds, but the AI for the bots and special infected is too poor for any sustainable joy. Versus is where it's at, and when it's good its pretty spectacular - whether you're a survivor being dragged off a few feet from the saferoom, yelling at your teammates 'just go, i'm done for, save yourselves', or an infected poised on a rooftop going over the plan on comms; hunter goes in first to give the boomer a few seconds to get close, jockey coming up from behind to nab a straggler and pull them around a corner or over a ledge... it's all good stuff, even when the plan inevitably disintegrates - the balancing is tight enough that one well-coordinated attack from the infected (or one fuckup from the survivors) can spell the end of the round, so there's always hope (or dread, if you're a survivor - even with your whole team at full health with medpacks to spare and the saferoom door in sight you know it could all go horribly wrong).
And yeah, L4D2 is a worthy sequal, tho i was on the ragewagon to start with. the complaint that they're too similar only really holds up if you're playing campaign or single-player - the new special infected completely change the dynamic of a versus round, particularly the spitter; in the first one it was too easy to survive a horde by holing up in a corner and shove/shoot/shove/shooting until the drums stopped. Now, not so much.
1 minute you go and shoot lonely zombies - then a bunch of fast zombies try to rush you - then you have another minute of shooting lonely zombies - then fast zombies rush you again - repeat until the end of the game.
Yeah, that's pretty much it in campaign mode. In versus, the ordinary zombies are just speed bumps, you barely pay attention to them as you're running through the level - you're too busy straining your ears for the gargling of a boomer behind a door, or looking out for the flash of green that means a spitter's about to ruin your day. It's hard to explain just how quickly things can go from rosy to utterly fucked.
So, yeah. Play it, but only if you can convince some other people to play with you, or you're willing to spend enough time slogging through pubs just to get the IDs of 3 other people that have a microphone and aren't retarded, so you can get into the good stuff.