I'm reading through the Tattooed Monk and I see the main issues with it (doesn't improve Unarmed damage aside from progressing at Monk level, Medium BAB, poor skills, and a wasted feat), but the benefits attached to the class are really... unusually high powered. Worth noting is that a Kobold can enter this PrC using Races of the Dragon fairly easily.
The tattoos themselves all require a Move action (not Standard) to use. I'm just going to give a quick breakdown of the tattoo abilities:
[spoiler]
Arrowroot: Lay on Hands, but Wisdom based. Class level*Wisdom modifier in HP can be healed. Assuming 10th level and a Wis of 30, that's 100 HP (vastly superior to Wholeness of Body, but not that great). Move action to use. Kinda meh.
Bamboo: 1/day/tattoo, get an Enhancement bonus to Con equal to the number of tattoos you have. Lasts 1 round/class level. Meh, not as good as the real deal. If it wasn't an Enhancement Bonus, it would be well worth it. The good news is it is usable 5/day.
Bat: As Bamboo, but Dex.
Bellflower: As Bamboo, but Cha.
Butterfly: As Bamboo, but Wis.
Centipede: 1/Week, Shadow Walk as the spell. Minimum of 5th level. Not worth the ability, considering a magic item in the Tome of Magic has the same effect and then some.
Chameleon: Alter Self 1/day/tattoo possessed. Lasts 1 hour/class level. Move action to use. Holy **** this is a great ability for a Monk!
Crab: DR 2/Magic per tattoo. Not worth it at all. If only it were a natural armor increase...
Crane: Stage 1 is immunity to Disease. Stage 2 is immunity to Poison. Stage 3 is Dragonwrought without the feat cost. Advances based on the number of tattoos you have, not on how many times you take it. Kinda meh.
Chrysanthemum: Fast Healing, minus the Fast. You heal 1HP/Class Level/Hour. Cuts healing costs nicely, but meh.
Dragon: 4d6 fire damage, DC 13. It's Scorching Ray, but it allows a save instead of an attack roll. Usable 1/day/tattoo. Due to an odd loophole in the wording, it's actually usable 3/activation as a Move action. Not worth it, but cool to think about.
Dragonfly: 1/day/tattoo, gain a Dodge bonus to AC equal to tattoos possessed for a number of rounds equal to your class level. That's +5 AC 5/day for 1 minute per use. Not powerful, but not bad either.
Falcon: Immune to Fear. Allies within 10ft gain your Cha modifier to Will saves against Fear effects. So much potential in the name, but not worth it.
Lion: 1/day/tattoo, Smite someone. +4 on the attack roll, +Class level to damage. Not red because it is a flat +4, not blue because it doesn't scale well and sucks for extra damage.
Monkey: Blanket +1/Tattoo bonus to Balance, Climb, Escape Artist, Hide, Jump, Move Silently, Open Lock, Sleight of Hand, and Tumble checks. So +5 at 10th level. You'd be on crack if you didn't say this was decent. Grant for a Skill Monkey (ha!) wannabe like the Monk.
Crescent Moon: Ethereal Jaunt 1/day. Requires 9th level. CL is decent, but uses/day suck without Action Points in the ECS. Not a bad choice, its just not as good as some new abilities.
Full Moon: +2 Luck bonus on Attacks, Saves, Skill checks, or Ability checks 1/day/tattoo. Move action to use, works only at night. Luck bonuses are hard to find, but its kinda underwhelming.
Mountain: +4 Con and Wis for 1 round/class level, usable 1/day/tattoo. -20 to Dex-based checks, but not Init. Blanket immunity to Bull Rushes or Trip Attempts. If not for the fact that you can't move from the spot you activate it, it would be amazingly powerful. You can't even take 5ft steps. The Con bonus is nice. The Wis bonus is nice.
Nightingale: Wholeness of Body. Better than the original because its a move action, worse than the original because it only heals 20 HP tops, and because the Arrowroot from above is 5 times better (no, really).
Ocean: Never need to Eat, Sleep, or Drink again. Meh.
Phoenix: Ick... SR 15+Class level. SR is fairly bad unless you can control it.
Pine: Remain Conscious? I think they meant Diehard. No such feat. Diehard isn't that good anyway.
Scorpion: 1/day/Tattoo, force an enemy attacking you to use their lowest ability score instead of Strength or Dex. Can be used during your opponent's turn without readying an action, and has no listed duration. They may have meant it to last either for one attack or one round, but RAW there's no duration listed. Check the Errata. Either way, this is a great way to screw someone.
Spider: Stunning fist required. Use a Stunning attempt to instead deliver a Contact poison. 2 Con initial and secondary. DC 10+Class Level+Con. If so many things weren't immune to poison, and didn't have a high Fort save, this would be great.
Sun: Full Moon's counterpart. +2 Luck to checks, but only works in daylight.
Tiger: +1 Attack rolls (untyped), +1d6 damage. Both apply to Unarmed Strike, lasts 1 round/class level, and usable 1/day/tattoo. Decent. If you can't think of anything else to take, take this and run with it.
Tortoise: 1/day/tattoo, replace your ranks in an untrained skill with your class level. Meh, kinda useful.
Unicorn: 1/day, reroll 1d20. That's it. Sucks.
Wasp: Haste for 1 round/class level, usable 1/day/tattoo possessed. In other words, Boots of Speed on crack.
White Mask: Blanket Immunity to any effect that would try to discern your alignment, Detect Thoughts, and Detect Lies. +10 on all Bluff checks.[/spoiler]
It isn't that powerful of a PrC, but it isn't that bad of a choice if you aren't concerned about your Unarmed Damage that much (there's other ways to boost it anyway). Also, if Epic Progression are allowed, Legacy Champion can be used to get more tattoos to work with.