We co-slept with our daughter until she was close to 2. She still sometimes comes into our bed and we're fine with that. For us, it worked great and I would do it again.
I have noticed that the negative experiences from co-sleeping seem to come one of the following:
1. one parent is against co-sleeping and the other is for it, so the whole co-sleeping thing becomes a source of conflict between the parents.
2. the parent (usually the mother) uses co-sleeping in an unhealthy manner. For example, the mom NEEDS her baby or toddler or older child to sleep with her to meet HER emotional needs. The problem is not co-sleeping, it is the mother's emotional neediness and unresolved issues, which no doubt shows up in other ways. However, I can understand how the adult who felt obligated to sleep with his/her mother as a child would associate co-sleeping with being "yucky."
Most co-sleeping parents I know have healthy boundaries and only co-sleep as long as the child wants to. In those case, the child seems to naturally move out between the ages of 2 and 5.
3. The baby or toddler is just not a co-sleeping type. Some babies, just like some adults, sleep best when they are alone. The slightest touch or noise can disturb and wake them.
From my experience and from talking to co-sleeping and non-co-sleeping parents alike, I believe co-sleeping works best when BOTH parents are completely on board with it and when both parents are putting the child's needs first.
I also want to add that while I thoroughly enjoyed co-sleeping with my daughter, I don't think co-sleeping is the be all and end all - there are so many factors that go into raising happy, confident children and that can be done if you co-sleep or don't co-sleep.
That's my 2 cents!