I had expected 5v output from my 12v supply but when I hooked it up to a multimeter I found I was getting just over 10v. This would fry pretty much everything in my circuit so I was pretty worried.
After a bit of research, I came across a thread on eevblog that mentioned a screw for adjustment. Looking closely you can see that there is a small screw on the side of the chip. Turning this quarter of a rotation anti-clockwise put mine at exactly 5.04 volts.
A few other small tips:
The thread seems to indicate that some models require a lot more turning (up to 30 seconds for one poster).
I had to push down fairly hard to get it to actually turn the part underneath instead of just the metal cover