Had no internet at home for a few weeks, and for some reason I've taken to playing l4d2 with bots (addiction being what it is). I tried to crack it on expert, but have only managed dead centre, hard rain and swamp fever (swamp fever finale was a royal PITA with the two tanks at the end). I didn't even bother to try dark carnival - I can only do the rollercoaster around 50% of the time on advanced.
Anyway, I've learned quite a lot about bot behaviour. It seems to me that their biggest problem is their inability to "run and gun", and also the way that they handle incapacitated players. For 'stand and fight' type battles the bots are actually pretty decent, although they still have the annoying habit of moving into your field of fire. But for run and gun (like the rollercoaster) they're pretty much impossible. They just don't keep up with you, and if one of the bots goes down they will all run back to revive him - even if he's back at the other end of the rollercoaster. The only way to proceed is slowly and carefully, which usually results in running out of ammo and/or slow wear-down of health.
The "revive at all costs" problem also makes them useless against tanks. If the tank takes down one bot, the rest will just run in to try and revive him, even if that takes them right through the tank. This, obviously, leads to rapid death of the entire team. They also sometimes screw up with specials. A number of times I have been strangled by an (out of sight) smoker, and had 3 bots just standing around me while I die, when they could release me at any time with a push.
Their insistance on healing you every time you drop to 39 health can also be problematic when trying to play on expert. Switching to the health pack every few seconds gets tiresome very quickly.
I think that the bots would be far more effective if you could issue simple commands, like "stay close to me", "heal me", "leave him down" etc. But yeah... I really wouldn't recommend playing with bots!
edit - that tank run looks awesome
