In previous years I have purchased both the Writers' and Artists' Yearbook and the Writer's Handbook, but last year decided to give Writer's Market UK a try.
I was so impressed, I bought the 2009 version, rather than going back to either of the other two. I feel there is more inforamation and the accompanying website (which is free to use) is invaluable.
Clearly, directories of this sort have to be compiled many months in advance of the year stated on the front, and so some information will be outdated by the time many people purchase the book. There will always be changes to the staff of magazines and publishing houses, and this will be an issue that affects all three of the main writers' directories, not just Writer's Market UK. Writers should ALWAYS verify contact details themselves, by checking a current issue of a magazine, contacting the publishing house directly, etc. This is also where the Writer's Market website comes in handy, as this is a resource that can be kept up to date on an ongoing basis.
It is a very chunky book - much larger than the other directories, which means it isn't the most portable. It is easy to find your way around the various sections, and it is very well laid out.
This is definitely my directory of choice and I will continue to be a loyal customer in the future.