Humerously enough I managed to prove myself wrong (about how the AI refuses to advance in the 'cover me' sequence), and I think I've really nailed the my prefered sensitivity settings now.
If you just stay very close to Garza, they won't throw grenades at you (strange, but this is normal and the beginning of the game) for about 40 seconds, a Helghast will appear by the door and start shooting at you, hell have a buddy on the lower level at the table corner, not quite able to provuide cover for him. If you go into the office area and kill the first Helghast, the other Helghast will file in 1 by 1 after that, sort of strange.
The AI did surprise and kill me once during the sequence, I think the AI can climb up a level as well as jump down, because After I'd killed one of the Office Helghast (the Helghast actually ignore Garza completely on this setting), I turned back to see Garza and ran towards him, only to have a Helghast CQC me out of nowere, maybe he was hiding around the corner and took an oportunity to chase me when my back was turned.
The AI IS actually good, very good in fact, I just don't think they were tweaked well for this sequence (or maybe it's just a difficulty setting thing).
Also, I've actually nailed my own personal preferences for sensitivity, and they're lower than default, believe it or not, about 25% for each. Personally I like it a lot better this way, because the weapons have weight, still, and I'm not actually contantly over ajusting my shots.
The problem with Killzone 2 is that it takes some serious getting use to, so for people who are like 'pass' keep trying it. Honestly, I think the final product will probably be great.
Oh yeah, I hung back from the APC firefight and much to my satisfaction, the Helghast actually managed to kill all ISA forces, and down all of my team mates. They then assumed a defensive position around were I was hiding after I made myself known to them. THey didn't do much beyond that, but I actually had two grenades thrown at me during the proceedings.
If the AI really is 'smarter' on the higher difficulty settings, than I might be in bliss on a higher setting or two. It's very important to me that the AI feel real and adaptive to your actions (it's basically the one thing that kept me playing Halo 3 imho). After experimenting more (this is about my 8th playthrough) the Helghast AI is there for doing things like advancing on tenemy positions, I would just like it if they were a tad more agressive in pushing you back.