Well, I've been through Left 4 Dead a few times now. If you keep trying long enough, you can eventually accidentally get on a good server. But you can never get on that server again, not on purpose anyway.
The difficulty is a little messed up, but that's to be expected from a game that's trying to provide dynamic difficulty. Hard is pretty easy. And Expert is crazy hard. One group I was in spent a couple hours trying to get through the second demo map on Expert. Of course, it didn't help that you start with whatever health you ended the last map with, so we always started wounded and had to waste time bandaging ourselves before we could start playing again. And fighting Tanks (giant bad guys) in the close quarters of the subway is almost 100% death. The winning strategy may be to play with one player always dead, as the game doesn't seem to spawn the worst baddies unless all four players are active.
Of course, this is all providing you're playing with competent players. Most people are used to standard FPS games and being the lone ranger. Going it alone gets you dead from the first boss you fight. But when you're together, you've just got a different problem.
Friendly fire is on in this game, and even when I wasn't playing with jerks who simply enjoyed killing each other, accidents were frequent. It's almost the best reason not to use a shotgun as one errant blast can knock a friendly out of the fight. But the best strategy is often to back yourself into a corner and kneel. That's pretty counterintuitive when facing a wave of ravenous undead. Most people run and jump and shoot like crazy.
And good luck if anyone in your group picks up a Molotov. Multiple times I found myself on my back, on fire, using my pistol to try and fend off burning zombies who were kicking me to death. It's fun the first time. :P
Overall, it's a pretty harrowing experience. And it only makes it worse that it's the same two maps over and over again. Returning to Brothers In Arms' hedgerow fighting will be like a picnic in an enchanted forest compared to this.
Also, I really don't see how this is a full price game. The online experience is sub par, and there are only four campaigns to play through. And I think you can only play two of those four with humans as the bad guys. And for all the supposed randomness, the encounters all just blur together after a while. I almost think I prefer the easier difficulties because they make boss attacks a change of pace. On Expert, you know there'll be another boss attack in about twenty seconds, so it's not really a surprise or very interesting. It's mostly just a grind.
Plus when I upgraded to the latest recommended video drivers, everything turned purple.
I think this is the first Valve purchase I've regretted.