You briefly gain vision of a nearby area.
Vision Radius: 4 (+1 per mutation level. At level 10, it reveals the entire map.)
Vision duration: 18 turns (+2 per mutation level)
Aside from the obvious applications, Clairvoyance allows you to target certain other mutations at long range and through walls, making it absolutely essential for a dedicated psionic character. Mutations that can be targeted through walls with Clairvoyance include Burgeoning, Confusion, Cryokinesis, Force Wall, Pyrokinesis, Stunning Force, Sunder Mind, Telepathy, and Teleportation. Most of these have fairly limited range, so you still need to be close to your target; but, of particular note, Sunder Mind and Teleportation do not.
Additionally, note that the duplicates created with Temporal Fugue will take full advantage of these synergies, provided you have Clairvoyance active before activating Temporal Fugue; in particular, duplicates with Clairvoyance and Teleportation will immediately teleport to enemies anywhere on the map in order to confront them.
Even without mutations that can attack through walls, Clairvoyance also allows you to target mundane weapons at a longer range than your light radius would normally allow, which can be important for ranged attackers.
At low levels, Clairvoyance reveals only a small area for a short time; this means you have to predict where to use it to gain the most benefit from it. At higher levels, this disadvantage disappears, allowing it to reveal the entire map indefinitely.
That aside, Clairvoyance itself has no disadvantages beyond the fact that it is a utility power that must be used with other powers or abilities for offense.