What’s the effect of pentagram

The Binding of Isaac wiki's got a pretty comprehensive (if slightly generalized, and minorly inaccurate) list. Remember that all cards are a single use item, so once they are used they vanish.

• O The Fool: Teleports the player to the starting room for that floor.
• I The Magician: Gives you the use of Spoon Bender (homing shots) until you leave the current room.
• II The High Priestess: Summon's Mom's foot stomp on a random enemy. Can also hit the player, and will do so if there are no enemies in the room.
• III The Empress: Gives you the use of Whore of Babylon until you leave the room. Whore of Babylon is Eve's curse, but unlike her version, this just hurts you half a heart then increases your damage.
• IV The Emperor: Teleports the player to the Boss Room.
• V The Hierophant: Spawns two soul hearts.
• VI The Lovers: Spawns two hearts.
• VII The Chariot: Acts as a single-shot use of My Little Unicorn.
• VIII Justice: Spawns bombs, hearts, coins, and keys around the player.
• IX The Hermit: Teleports the player to the Mini-boss Room, or the shop if a mini-boss room is not available.
• X Wheel of Fortune: Spawns a Slot Machine.
• XI Strength: Makes the player bigger, increasing maximum health and damage until the player leaves the room.
• XII The Hanged Man: Gives you the use of Transcendence (flight over gaps, rocks and spikes) until you leave the current room.
• XIII Death: Gives you a single use of The Necronomicon (massive damage to all enemies in the room).
• XIV The Tower: Gives you a single use of Anarchist's Cookbook (spawns multiple bombs around the room at random).
• XV The Devil: Gives you a single use of Book of Belial (increased tear damage) until you leave the current room.
• XVI Temperance: Spawns a Blood Donation Machine.
• XVII The Stars: Teleports the player to the Item Room, or acts as Telepills if there is no item room (i.e. in The Womb and Sheol).
• XVIII The Moon: Teleports the player to the Secret Room.
• XIX The Sun: Completely heals the player, does damage to all enemies on screen and acts as a Treasure Map and Compass for the current level only.
• XX Judgement: Spawns a beggar. If you have no money, spawns 1 heart, coin, bomb and key instead.
• XXI The World: Acts as a Treasure Map and a Compass for the current level only.

Tarot Cards II, III, XVII, and XIX were added in a patch on November 1 2011.

TL;DR: Yes, try every pill you find

In the game, according to the list near the bottom of the items page on the wiki, there are

8 pills with definitely good effects:

1. Bad Gas: poisons surrounding enemies
2. Balls of Steel: gives 2 soul hearts
3. Full Health
4. Health Up
5. Pretty Fly: adds a close orbiting fly
6. Range Up
7. Speed Up
8. Tears Up

4 pills with definitely bad effects:

1. Health Down
2. Range Down
3. Speed Down
4. Tears Down

And 5 pills with neutral effect or possibly good or bad effect:

1. Bad Trip: Take 1 damage if more than 1 heart left, otherwise grants full health
2. Bombs Are Key: Swaps bomb and key counts
3. Explosive Diarrhea!: Drops lit bombs while walking for a short time
4. I Found Pills: no important effect
5. Telepills: Teleport to a random room

If we assume that the set of pills for each playthrough is uniformly chosen from all of the pills, then it would be wise to take every unidentified pill because it is more likely to be good than bad, and even some of the neutral pills can be useful in the right circumstances.

If we assume a reasonable bad distribution, with 2 bad pills, 2 neutral pills, and 2 good pills (generally worse than random but possibly what actually happens), it may still be wise to take random pills. Some of the neutral pills have useful effects at times, so it would be best to try random pills with as many of the following conditions as possible

• You have 1 health (so Bad Trip heals you)
• You want to switch bombs and keys, or at least wouldn't care if they switch (Bombs are Key)
• You want to drop bombs in the room, possibly to break rocks or look for the secret room (Explosive Diarrhea!)
• You would be OK teleporting into any room in the level (Telepills)

So the absolute best time to test pills would be when you have defeated the boss and cleared the level and you have 1 heart and you still need to break a rock somewhere.

However, even discounting the possible benefit of neutral pills, I would say it is still good to test every pill because knowing what pill you have and what pills there are can be useful. If you know that you have an Explosive Diarrhea! pill for example, you can use it against a boss or a room full of enemies that are easier to kill with bombs.

Also, if you know what bad pills there are, you can more accurately determine whether to buy the PhD or Lucky Foot, because they transform bad pills into the corresponding good ones. So if the bad pills are Range Down and Speed Down, and you already have both of those at max, it is probably not worth it to buy the PhD because you will not get any new benefit from those previously bad pills.

Thus I conclude that unless we know that we can get very bad sets of pills, it is probably good to try unidentified pills.