I want to check reviews when deciding to buy a lens. There are so many factors beyond what is written in the name. Where can I find such reviews? What information does each site cover? Which reviews are most comprehensive? Which sites do the most meticulous scientific measurement, and conversely, which sites cover important subjective factors which are hard to put a number to?
Note that in general, comparing between mounts is not a good idea. (Even comparing lenses on a different shared mount than your own is a bit better.)
This is a "community wiki" post, where answers do not have individual ownership. Please use the answers to rate and comment on each site.