Frag grenades have very short range, moderate aiming time, and a one second delay before exploding in a 3x3 area, causes 40 damage to items and creatures, 160 damage to structures and terrain features. It's main benefits are the strong damage versus structures and large concentration of hostiles in tight areas. It offers a viable combat alternative to those with sight deficiency as it hits nine tiles at once, reducing target "miss" chances.
Usage without oversight is prone to vulnerable positioning, inefficiency against moving target and cause (some times) unwanted collateral damage.
Hostiles equipped with frag grenades are very dangerous, specially against your Improvised turrets and drafted (standing) colonists in cover, mostly because their casualties don't matter unlike player's colonists and will just charge forward without any care, but of course will retreat under fire. Players should pay attention to the "throwing" sound and clear the area fast enough to avoid getting caught in its blast radius.
Frag grenades are unlimited and are thrown with the same animation as of shooting, they cannot be used over walls and must have a direct line of sight. A circular zone will generate to indicate range, then another 3x3 indicator with a circular center will become the aiming spot. The character cannot hold both the primary weapon and the grenades at the same time but one only.
Frag grenades produce blast explosion but no fire, allowing instant extinguishing of fires.
Frag grenades can also be used to clear dumping zones, though if you are clearing corpses it's better to light them on fire instead.