For some people, like my son and myself, our bodies seem to react to/process HFCS differently. Both of us have hypoglycemia. I think most people don't have these wild reactions to HFCS, but I do believe people with blood sugar issues - like Bumbershoot's DH and MIL, my son and I - have different reactions. I'm not sure what the connection is, but I do believe there is a connection.
If I eat/drink something with HFCS, I feel very nasty, very witchy, want to scream at the DH and kids for no good reason, feel very stressed out.
If I eat something with sugar - for example, cookies I've made from scratch - I do not have these issues, although I might get hungry again within about 2 hours.
If I eat a piece of fruit by itself - for example, an apple - I will be very, very hungry within an hour, but I do NOT have any rage issues from it. Interestingly, if I eat a salad for lunch - even with meat on it - within an hour, I feel like I haven't eaten any lunch yet.
I think for some people, HFCS is very much an evil to be avoided. My DH and other son don't have any problems with it, but avoid it since I won't let it in my house. My niece, who is hypoglycemic, gets very nasty/rage filled after eating it.
Bumbershoot, good luck with MIL!