My handwriting is nearly illegible, even to me. I think the main problem is that I try to make writing as efficient and fast as typing which results in blobs that don't necessarily look like words. I still prototype stuff on paper, but if I haven't looked at a diagram in a while I sometimes have to guess as to what I wrote.

That being said, I have the ability to provide very nice handwriting, but it requires an obscene amount of time and it hurts my hand after a while.