I finished all 4 campaigns (3 on Advanced, 1 on Normal).
I don't plan to finish a campaign on Expert (too frustrating).
I've played through both Versus campaigns several times now. I've recently stopped taking Versus mode so seriously, even going so far as to call Left 4 Dead Versus the "Mario Kart Wii" of first-person shooters. (For those not familiar with Mario Kart Wii, MKWii and L4D are both too random to be considered a true test of skill.) As long as I don't take the game too seriously, I tend to have a good time.
My decision whether to play Team Fortress 2 or Left 4 Dead usually depends on what other ToJ members are playing. If more are playing L4D, I'll play that. If more are playing TF2, I'll try to round up a few and get the ToJ TF2 server hoppin'.
I'll also opt for L4D if I don't feel like spamming half my Steam Friends list in an (often vain) attempt to gather people on the ToJ TF2 server.
Once the next TF2 update drops, I think we'll see players swarm back to TF2--at least until Valve releases an update for L4D.
Oy, we first-person shooter players sure are fickle people, aren't we?
EDIT: I think I'll be playing TF2 far longer than I'll be playing L4D, but it's too early to tell. I think TF2 has much greater lasting power.
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ROFL