Quote Originally Posted by Didz
I hardly bother with assassin's unless I intend to launch a mass terror bombing campaign on an enemy city, but spies are vital.

I keep at least one in every city to fend off enemy spies and use one with every merchant travelling abroad to spot potential take-overs. They are also useful scouts for my armies letting me know in advance what opposition and benefits I am likely to face. Though, to be honest I rarely expliot the 'open gate' option as its usually too expensive to try and force troops through.
I tried hard not to say "agents are useles".

If spies help you take cities, and you are acting consistent with the muster armies, take cities strategy you should be fine in terms of winning the game before time runs out.

I don't tend to bother with spies to find out what enemy troops strengths are. I just go to the neareast city expecting to beat whatever the enemy has. Maybe I could do better if I micromanaged my agents, but I don't and win relatively easily.

If someone is doing a great job micromanaging agents and having trouble winning, they are probably being distracted too much from the basics. Place more emphasis on raising armies and attacking cities with them.

This is not to say you should ignore the other aspects of the game.