Agility is not indicated entirely by the inertia modifier. According to this wiki article agility is a product of mass times the inertia modifier of a ship.
Based on this forum post:
The explicit value of agility isn't all that important to know when piloting a ship. The reason agility increases your alignment time is due to the fact that it increases the ability of your ship to turn (i.e. align with warp target). This also reduces the amount of speed you lose while turning and gives you a tighter (and faster) orbit.
Let's look at a Slasher (frigate) vs a Mammoth (industrial), which are both Minmatar ships.
Given that Agility = Mass x Inertia Modifier:
Slasher: Mass 1.075 million kg; Inertia Modifier 2.83; Agility ~3.0
Mammoth: Mass 12.5 million kg; Inertia Modifier 1; Agility 12.5
So you can see that the Mammoth is going to turn a lot slower than the Slasher (as it should). There are also skills, modules, and implants that can affect Agility, so you can't be 100% certain what someone's Agility will be.
*Advanced Spaceship Command
Skills with a * denote skills that only affect ships requiring the skill to pilot them.
The implants list here is old, but it says that there are slot 6 hardwiring implants "Hardwiring - Eifyr and Co. 'Rogue' AY-" which modify agility. They aren't in the list of implants here though.
It probably won't be worth using these implants, depending on how expensive they are and how much you'll actually get out of them.
Inertia Stabilizers reduce the inertia modifier of your ship, which reduces your overall agility. Do note that you can have more than one inertia stabilizer, but their effects will be reduced. You should really only use one if you are going to use one at all.