X-ray Vision is a mutation found in the abandoned complex. It allows you to see into people's inventories and into closed containers. Additionally, your ability to see through junk gets you +1 to medic, scavenge, and locksmith. However, you have trouble seeing in the dark.
The skill buffs are nice for brainies, and the x-ray vision is useful for checking if people are carrying valuable junk (or if it's worth smashing a locked container).
The success of your x-ray, and how much you are able to see of a person inventory depends on how much clothing they are wearing.
As a bonus, you can see the @nudes of other players by simply looking at them.
It lessens your ability to see at night, making areas with low-light appear completely dark. Keep in mind that this is a LOT of areas.
This means that you'll have to carry a light source (lantern, night vision goggles/tac helm or rhodopsin implant) with you 24/7. Not a huge issue for non-Freaks; rhodopsin implants are cheap and widely available.
Overall, the stat buffs are small and the active ability is useless - you already see what people carry by looking at them and containers are smashed open just as fast as typing "xray cabinet".
No appearance change.
(during use:) User squints up her/his/its eyes and peers intently at Target.
(used on:) You feel a strange tingling sensation in your belly.