The "experts" will all tell you that if it's done right, it shouldn't hurt. I think they say that to try to encourage women to do it, since it's the healthiest and less expensive way to feed a baby..... That being said, I think that while it may be true for most women, it's definitely not true for all, and health care providers/advisers would do more good by being honest that its sometimes uncomfortable, sometimes even pain, and it can take a while to work through the issues.
I had constant pain for the first 6 months, though the first 2 months were the worst. I saw a lactation consultant 4 separate times, after seeing one in the hospital several times. My son has a high palate, an overbite, and was slightly tongue tied. So, um, not be be graphic, but my headlights no longer point straight ahead any more (ahem...). I wasn't doing anything wrong, but the shape of his mouth was making it hard. I also think that this is one reason that it took him forever to nurse (like, 40 minutes per side when he was a newborn/infant, and he'd nurse both sides; this gradually decreased over time to about 15 minutes per side). I'm also extra sensitive, and for months, with every letdown, it felt like I was being stabbed in the nipple with a pin. That finally went away over time as well.
I finally read an article by a doc who blogs (Dr. Amy something; she blogs about birth & related topics) who basically said that doctors & lact. consutlants, etc., who promote breast feeding are doing a disservice to women by not admitting that sometimes, it just isn't the blissful ideal that it's portrayed as, and that there's not necessarily anything wrong with that. When I read that, I finally stopped blaming myself for doing something wrong, and instead gave myself a pat on the back for keeping going even though it was pretty awful for a while.
For me, I found that, especially at first, varying the baby's position, with every feeding helped to prevent one part of the nipple from getting stressed too much, and sort of spread the stress out. That was how I managed to avoid getting cracked nipples again. The other thing that I found helped was to disengage the baby, rather than let him "slide off" when he was finished; he tended not to release very well at first, so that caused extra pain. I broke the suction and took him off the breast, rather than letting him relax off on his own.
I also used a lot of lanolin. I mean a lot. My 2 SIL said, oh, I still have most of my 3 oz tube left. Well, I put lanolin on after every single feeding, and I've used up almost 2 tubes in the course of nursing my son.
If there is cracking/bleeding, clearly there is something wrong. But even in a case where it's been tough the whole road, I can say that for us it was worth it. It did get better, and while for us it was never the blissful time that so many tout it as, it was very worth doing. And no, I'm not a martyr. LOL. :)