Our family decided to give my son all of his immunizations on schedule except for his MMR. We delayed his MMR until he was 2 1/2 years old (I was registering him for preschool and he had to have the shot to be registered). We delayed it because of autism concerns. Our pedi agreed to delay it because he is a stay-at-home baby so his exposure risk to disease is greatly reduced.
Ironically, seven days after receiving his MMR he had an "atypical febrile seizure". I did some research and found that this is a possible side effect from the MMR. He just had a second seizure yesterday. I don't know if the MMR was his trigger or not but it does make me skeptical.
This might be more that you were looking for. By the way, if you chose to delay his MMR, make sure you double check with the nurse before she gives your son any shots. Even though my pedi and I had agreed to delay, the nurse still walked in with the MMR in hand ready to give to him. If I hadn't double checked exactly what she was injected, he would have received the shot anyway.
Good luck!