It is! Its like being pinched, but over a long time. But even though it hurts, its all worth it in the end.
Remember that in some tattoo shops, they will tell you, that you cant move or breathe in a few seconds at a time, because when you breathe, your chest moves, and the tattooist can make a mistake.

From what ive seen its VERY painful. I watch miami ink w/ kat von d, and she wants to finish the angel on her ribs but she says it hurts too much to do the whole thing. but again she is super skinny

It definitely hurts More then alot of other tattoos!

i got a new tattoo on my back, it is on the left side and goes from my arm towards my ribs... ahm ok.. bad description haha : D .. but it didn't hurt .. in no way! i could have played cards or even drunk a cup of tea .. i didn't even feel the needles, and i'm really skinny.. maybe it depends on you