I think the key to making interesting AI is to make the variables that drive the AI like accuracy randomly fall within a "playable" range. So if play tests show accuracy between 30-50 is "playable" then create a script that randomly sets accuracy for each encounter (maybe even target specific range based on the players ladder rank) For example the AI sites an enemy and before it fires its weapon the AI is reset randomly choosing the levels of the variables that drive the difficulty of the AI character. To make this matter, animation needs to depict the changes as a "reaction" to this change. Even something subtle like changing the idle silhouette enough to make it clear the AI has been affected by the situation, just like players. This is using the same system that dice rolling RPGs use and it adds a fair amount of luck to the gameplay.

After playing for more time player come to know about AI logic. So it's more fun to play with humans than AI.

Any one can experience this while playing Counter strike and can easily find the difference by playing game like Virtual Cop.

When multiplayer games were not there at that time playing Virtual cop was fun, but after playing multiplayer games I prefer them only.

There is no repetitive action or the same logic. Its fun to play.

Something I often find fun about playing against humans vs. AI is unspoken diplomacy, personal emotional plight and the big picture. When one player is doing well often the underdogs will turn their cross-hair from a weaker enemy to the stronger one even when they have a clean shot on a weak target. This moment of hesitation to possibly lose the match though fight for the greater good could be a fine addition to the AI world.

Seeing AI think about the fight is important in my opinion. Because everything everyone does in most games is pretty repetitive when it works. But if an AI character clearly changes its mind (even at its own peril) because of the bigger picture or because of a clearly "personal" situation it feels more like your playing with something that isn't just a part of the game, it feels like its something your playing the game with.

Something I often find fun about playing against humans vs. AI is unspoken diplomacy, personal emotional plight and the big picture. When one player is doing well often the underdogs will turn their cross-hair from a weaker enemy to the stronger one even when they have a clean shot on a weak target. This moment of hesitation to possibly lose the match though fight for the greater good could be a fine addition to the AI world.

Perfect Dark did some simple but interesting things with AI opponents.

They had: JudgeSims, VengeSims, KazeSims, PreySims, CowardSims, FuedSims, PeaceSims, and one or two less-behaviorally interesting sim types, aside from difficulty levels.