I really think Hl2 AI was pretty good, just the City 17 atmosphere didn't give it a good chance to show itself. For example, turn on Garry's Mod (Assuming you have it), go to Gm_Construct or some reasonably open map with some buildings, and spawn a large group of Combine. In the 5.7 seconds before you die, you'll notice that they split into groups if you're trying to hide somewhere, and try to flank you. If you're in a building, most will storm the main approach, trying to overwhelm with numbers, while some will search for a back door to hit you in the back. They'll also use any reasonably sized cover once they're done advancing and start fighting you. Therefore, I think that the Ep.2 AI will be much improved, as it offers enlarged spaces. Though, I think Combine really need to show up in larger groups to be a good challenge (of course, I'm a noob who plays HL2 on Easy, so what do I know?)
I agree picard, it usually isn't given enough of a playing field.
I'm in the process of playing the Half-Life series through in order on the highest difficulty setting, starting with HL1 and it's expansions on difficult, and now Half-Life 2, I'm up to Nova Prospekt now.
Anyway, as I was playing Highway 17, I had a number of encounters with Combine soldiers in and around the buildings on the coast, now I tried somewhat unconventional tactics for taking them down, rather than just assaulting them outright, I would park the buggy some distance away, walk, and then sneak in without anyone noticing, then I'd play cat and mouse with the combine troops, making flash appearances in doorways, firing a few shots, and then bolting for it to see how they respond, then do it all over again, leaping through windows and climbing on rooftops... Well anyway, in the process of doing so I noticed a lot of interesting AI behaviour, including fire and movement, regrouping and charging at me together, staying in cover then all popping out to shoot at me at the same moment, coordinating their efforts. The AI really does look a lot more competent in situations like this, where they have many ways around, different routes to take and ways to flank their target (you). Needless to say, I was quite impressed with the AI here.
The reason for that is because most of the AI is actually unscripted. Allowing characters to have multiple areas to move into (out of, behind, above ect.) it allows every battle to play out different, and they will use the enviroment to their advantage. And yeah, those guys are really smart.
Once when I barricaded door with alot of random stuff from house, they just put a grenade onto it and broke it to shreds. I realy liked that one.:)
When the Combine throw a grenade, they throw them not just as an attack, but to force you out of where you are, or to distract you. I always get a sense of urgency when that happens.