PNO3 just got repetitive for me. Not enough new machines to fight, and the fact that I had to slam the A button as fast as possible to shoot just sucked. Yes you could upgrade it, but even fully upgraded it was still faster to just mash "A". Bosses were a breeze with the power drive, and yet at times in normal rooms I felt when I got killed it wasn't because I was bad, but because the game cheated.
The similarities to RE4 are uncanny though:
Moody music, no moving while shooting control scheme (I never had a problem with this in either game), buying and upgrade shop, enemies drop chash, a "combo" scheme similar to what RE4 does with mercenaries, a graphically detailed main character.
Where PNO3 really fails is its lack of variety. The same enviroments and enemies just get old. It also doesn't have all the bells and whistles of RE4: cutscenes, graphical details, a lengthy quest, ect. The controls could have also benifited from the "Splinter Cell" view while aiming instead of the current lock on and mash "A" configuration. The game pretty much sucks, and you shouldn't have to play for 6 hours before it begins to get fun. I did, and still lacked in fun.
edit: oh the regenerators in RE4 got me completly rehyped for Killer 7. Bring it!