Good

• ADHD: Increased movement speed.
• Eidetic Memory:The last known location of enemies is shown on the map.
• O.C.D.: Breaking items will restore 1MP each.
• P.A.D.: Peripheral Arterial Disease. Your legs have no pulse, so you do not trigger spike traps.

Bad

• Alzheimers: Means that your Map is replaced by ???, strangly your Mini-Map in the top right-hand corner is unnafected by this and will remain unchanged [Not known if this intentional or unintentional].
• C.I.P.: Congenital Insensitivity to pain. Health bar is no longer visible.
• Color-Blindness: Means that your screen is in Grey, White or Black. This can be confusing as backrounds can blend with enemies.
• Dementia: The player will occasionally see enemies that do not exist. They cannot be damaged, and cannot damage the player.
• Dyslexia: Scrambles letters in text.
• Farsighted: Everything nearby is blurry.
• Hypochondriac: Damage shows as considerably higher than it actually is.
• Nearsighted: Everything far away is blurry.
• Nostalgic: Sepia-tones the game. The loading screen will also say "Reminiscing" instead of "Building"
• Stero-Blindness: Means every enemy and your self do not have sides and will look like a flattened version of themself when turning sideways. (Can reduce visibility of mage enemies in some cases.)
• Tunnel Vision: No peripheral vision. When enemy projectiles are fired outside the player screen range, they won't show up in the borders of the screen as they usually do.
• Vertigo: Welcome to Barfs-ville. (Everything on the screen is upside down)

Could be Good or Bad

• Alektorphobia: The drumsticks that normally drop will instead run around and attack you. Once killed, they will heal you as normal.
• Ambilevous: Any spells will go the opposite horizontal direction from their normal direction.
• Dextrocardia: Your HP and MP pools are swapped. Who knew? (HP and MP swapped)
• Dwarfism: Makes you incredibly small and gives you an advantage as you can access most secrets in the game, but you will have a much smaller swing range.
• E.D.S: You are very flexible. Allows you to turn around while swinging your weapon.
• Ectomorph: Increases knockback caused by enemies.
• Endomorph: No knockback caused by enemies.
• Gigantism: Makes you incredibly large, this can be bad as your hitbox is much bigger but also good as your swing range is much much large giving an advantage against enemies with less range.
• Hypergonadism: Greatly increases knockback caused to enemies.
• Muscle Weakness: Enemies don't get knocked back.
• Savant: Each time you use your spell, it changes to a different one.

Cosmetic

• Baldness: Makes you bald, hence having no feathers on the helmet. Also makes the loading screen for the Castle say Balding instead of Building.
• Coprolalia: %#&@! (You swear when you are hit, your last words are always punctuation)
• Gay: No effect, bar reversing which statue (knight or lady) grants the chicken leg or mana potion in waypoint rooms.
• I.B.S: Stands for Irritable Bowel Syndrome and all it does is make you fart and leave a cloud behind when you jump.

The headlight itself lets you see better in dungeon area, but it is not mandatory really, you can even simply increase your screen brightness for a similar effect.

But the Spelunker upgrade also grants your miner the passive ability of locating rooms with treasures, this ability doesn't cost mp and has nothing to do with headlight.
So even though headlight isn't really useful the Spelunker upgrade is probably worth buying.