Hi, I have the Canon EP-EX15 Eyepiece extender on my 5d2, it is the best thing I never leave home without out!
Everything works as normal except that its 30% smaller view, on a FF its not an issue. I have the same problem seeing the whole of the frame but this little devise solves the problem and you don't have to squash your face up against the lcd/eyepiece and no ugly nose oil stains on the lcd, its also cheap and I don't wear glasses! all good!!!