The Guardian Squad is the Eldar "backbone" ranged unit, essential from Tier 1 to mid-game and still very useful even in Tier 3. It is basically a ranged anti-light-infantry squad that seems to have trouble standing up to heavy infantry units, but has quite a bit of utility.
The un-upgraded basic squad is very fragile, losing models (30 each) rapidly and often destroying an unskilled player's economy through Requisition bleed. However, upgrading the unit with Battle Equipment yields a passive bonus of +33% health – reducing model bleed in itself – and the Energy Shields, which (with some extra micro) allow the unit to absorb significant amounts of ranged fire without losing any health at all, and Fleet of Foot gets the squad away from danger before losing models. The Tier 2 Warlock squad leader grants a further -17.5% ranged damage mitigation, actually making the "fragile" squad quite durable.
The basic damage output of a full squad is 43.75 DPS of piercing damage, which equals a Tactical Squad. Of course, the Guardians will lose a 1-on-1 shootout with Tacticals (who have more health and take only 67% of the base damage due to heavy infantry armor). Adding the Warlock will increase the squad's damage to 59.39 DPS, and activating Embolden further enhances this to 74.24 DPS. This makes a group of 2–3 Guardian Squads very bad news to light infantry.