As Kixeye does for its other "system" specials, they have combined two specials into one to create a useful combo special, saving you slots. The Agility System combines Guidance Scrambler, boosting Evade, and Shielded Electronics, helping Stun/Slow resistance.
FYI: Stun resistance decreases the time affected by pinch rockets. Slow resistance decreases the time affected by disruptor weapons.
(click table to expand)
The table above shows the relative weight and effectiveness for the system. It's like combining GS2 with SE2.5. The weight is pretty good for what you get - the drop in evade from GS3 isn't too bad and it weighs less. Of course, if you don't need the slow/stun resist then you might as well use GS2 instead of GS3. The painful part is the long build time. SE3 was already a long build at over 20 hours. Most level 3 specials take around 12 hours, even "systems". Other long specials to build are Siege Targeting (48 hrs) and Hydrodynamic Shells (27 hrs). But if you need the evade AND the resist, agility system is a great choice - special slots are tough to come by.
USE IT FOR: Blitz base fleets (yeah, I'm looking at you Hellstrike), anti-reavers, maybe any other ballistic fleet.
FORGET IT FOR: If you want to save some weight over GS3, just use GS2. Creeper base fleets will stay out of range of player disruptors.