This article is from the X-Men Comic Books FAQ, by Kate the Short (firstname.lastname@example.org) with numerous contributions by others.
Here it is, in Paul's own words:
A+ - unequivocally recommended.
A - highly recommended.
A- - Could be better, but still recommended.
B+ - Worth a look.
B - Fine if you like that sort of thing.
B- - Passable.
C+ - Unsatisfactory, though not actively bad.
C - Badly flawed; for fans only.
C- - Bad; for completists only.
D+ - You'll wish you hadn't bought this book.
D - You'll wish you'd never even read this book.
D- - The creators wish they'd never even read this book.
It should be noted that the review grades vary depending on a number of
factors, including Paul's mood when reviewing the title. And no, there is
*no* grade of F. The UK grading system generally doesn't use F; work
that bad isn't accepted for a grade at all. Paul doesn't, hasn't, and
won't grade a book F. Perhaps a book so vile doesn't exist. If it does,
Paul certainly wouldn't bother to review it.