DD has a family -- and personal -- history of MMR reactions when the vaccine is given to younger children (say, under 12 or so). The same reaction has occurred in myself, my brother, and both of my children. According to my mom (who had wild measles as a child), the reaction is equivalent to having the actual disease. It consists of:
High fever (also febrile seizures and delirium in DD)
Lasted about a week or so.
We *think* (but we're not sure) that the reaction has something to do with the large amount of American Indian blood we have. This is true for everyone who has had the reaction, but not for my mom... it's from my dad's side, and he had a life-threatening case of wild measles as a child. Of course, it's common knowledge that measles was one of the diseases that led to many, many deaths among American Indians when they were exposed to it.
MMRs given at a later date (post-puberty) don't seem to have a strong reaction in my family.
Here's the issue, though. We can bet that if we give DD the second MMR while she is still young, or we get it done on DS when he reaches the usual age, we'll be dealing with this big, nasty reaction. On the other hand, if we DON'T give the MMR and one of our kids somehow gets wild measles, it'll probably be much more severe and possibly life-threatening.
Current plan is to get it done but not until they are older (say, 12 or so), since the reaction is a lot less severe in the above-mentioned family members when the MMR was administrated at a later age.