personally, i could never use CIO with a baby, but a toddler is a different matter.
for me, it made a huge difference when i KNEW the ONLY reason the child was crying was because he/she was angry.
once i was confident in that, i could let them cry.
HOWEVER, with my daughter, who's now almost 3.5, i have a slightly different approach. when she gets REALLY tired, she goes a little crazy. starts crying/screaming etc. after 5 minutes, i go in and ask if she's ready to stop crying and have a cuddle.... if i don't get an answer, i leave. i continue to do this in 5 minute intervals. after 2-3 times, she stops crying and says, "yes, please". at which point, i sit on the side of her bed, she crawls onto my knee and we give each other a big, strong hug. the hug sometimes lasts 2-3 minutes. during that time, i talk quietly to her, preparing her for the end of the hug at which time, she must lie down and go to sleep.
this ALWAYS works with her, but it would not have worked with my son. for him, i just had to let him cry.
1) your child will have forgotten all about the crying by morning and certainly WILL NOT hate you! if you can't manage this now, it will only get worse...
2) you CANNOT give in once you have committed to the process. YOU are the parent. you cannot let the child call the shots. if you let your child cry for an hour and THEN you decide you can't take it, you've just taught your child that IF he cries for an hour, he'll get what he wants!