I bought this book as a replacement for Penrice, but, for me, it's not as good.
The layout is a little confusing and some words don't seem to be listed, although this may be the result of my not being able to find them.
On the other hand, if you are not confident in your Arabic, this dictionary is superior to Penrice's, since it contains an alphabetical index, an index of roots, and a lot of information on pronunciation.
Penrice's book was also written by a non-Muslim Victorian scholar and restricts itself to strict definitions of the meaning of words. Omar's dictionary has more commentary and details.