I use EXP because it lets me write about 25% faster than I write by hand (with a bunch of macros and keyboard shortcuts I don't even the mouse to click on various symbols, like most WYSIWYG editors).

http://www.expswp.com/

I wrote the following in about a cumulative hour using EXP:

http://math.uic.edu/~robertk/files/mth504_notes.pdf

I've used and seen many people use various math typesetting editors, and from my experience EXP lets you be the most productive (not good for research papers, but very good for writing homework, notes, etc.).