This really depends on age of the kids in the share. My DD has been in a share since 4 months and will stay until 2.5 when she starts school.
The good: socialization, learning for your child in a more intimate setting, shared cost, ability to learn from another family, having two places for the children to be (we swamp hosting every other week - it's like all new toys!), built in playdates
The bad: compromising on schedule, ensuring child's developmental needs are met if both children aren't on the same path, needing to have the same parenting style as the other family / nanny
In my opinion and experience, the good far outweigh the bad. It is definitely dependent on experience and finding the right match. I've heard many bad experiences as well, but mine just worked.