Hi E.,
We've never let our son cry without comforting him. We actually still do this and he's 3 (though he doesn't cry much). There is a lot of support for not letting a baby cry, and research that shows that supported babies and children are MORE confident and better behaved than others who are not.
Unfortunately, some babies are just not *good* sleepers, and no one knows why. What you can figure out, though, is any tricks that work to help your baby sleep, and then the baby is happier and more relaxed and you can eventually transition to those easier ways of getting the baby to sleep. One big thing that I know people miss is the reflux (which I have myself right now, being pregnant). So you might try a sleep positioner that inclines the baby.
My son needed help falling and staying asleep also. The tricks with him were to help him fall asleep by nursing, bouncing, wearing, or whatever,(but not too deeply), then putting him in the crib, and if he started to wake up, I'd pick him up quickly and get him back to sleep again. Same with waking early from a nap, I'd just help him get back to sleep again before he really woke up. I also had a short pile wool mattress pad in his crib, which I think helped the crib not feel so cold and crunchy to him. Whenever I could, I took advantage of his nap to co-sleep and take a nap too. It helped me function better, and he slept better too.
If you read about our sleep patterns, as adults our sleep cycle is to come nearly awake every 90 minutes. As babies neurosystems are developing, it makes sense that their sleep cycles are also bringing them nearly awake at intervals, and that they are just waking sometimes. In the No Cry Sleep Solution, she encourages parents to help children be able to fall asleep in different environments, which is kind of against convention. I think that different babies must need different things, and that if you follow through on some things that show signs of working, you'll find the right one; maybe your baby feels better with one or two ways of falling asleep. It does take a bit of patience and time, like a couple of weeks once you hit something that shows signs of hope.
By the way, our son is an awesome sleeper now! Since he was about 18 months, he sleeps 11 hours, and on weekends he sleeps till 8. Ahhh. It'll happen.
Best wishes!