These are the best unaligned archers you can have. Though they have only half the unit-strength of regular archers, they each shoot two arrows and have accuracy better than even Master Longbowmen. In addition to a much richer perk selection, Surprise Shots come base, potentially greatly increasing their damage near the beginning of engagements.
Because of their higher cost and upkeep, it will be very late game before you could potentially replace all archers with them. A good cost effective mix is 2 units of Master Longbowmen and 1 unit of Deepwood Archers in every army.
Master Longbowmen research is a prerequisite to researching the recruitment of Deepwood Archers.
Unlike some units, Deepwood Archers have no useless perks. Master of Ambush, while very useful is only helpful to players willing to devote some micromangement to his archers.
Magic resistance is particularly helpful against armies that use Old-Faith spells as Curse of Shadows is particularly suited to hitting archers, often destroying multiple units in one cast. Sharpshooter also helps counter Old-Faith armies, as it is a particular favorite tactic for them to cast a weather spell, which their achers are immune to.
Without needing much finesse, Regeneration and Rain of Arrows are simply useful all-around.