In January I crisscrossed Cambodia for four weeks. The two guides I took were Lonely Planet and Rough Guide. Both are worthy guides. This guide was at its best when it came to Siem Reap/Angkor Watt. Its coverage of Angkor Watt and the multitude of other temples is excellent.
Both guides will give you a great orientation of Cambodia. They will point you to the best sights to see, but Rough Guide gives you a better understanding of the history, religion and culture of this country.
It has good maps, reliable recommendations of what to see and do, a good history/religion section and a very good 'recommended books' section.
Cumbersome, with all Rough Guides, is the price code@ system for accommodations, ie. 1=less than $5, 2=$5-$10, 3=$10-$15, etc. You need to memorize the amounts or flip back to codes. Lonely Planet is easier to use; it tells you what it cost in dollars - Duh, what an idea!
Overall, if you are going to explore this little known, astonishing and wonderfully mysterious country, you will do well with this guide. I recommended watching two excellent DVDs before you go (Pol Pot -1997 & The Killing Fields 1984) and also taking The Gods Drink Whiskey@ by Stephen Asthma, excellent. Happy Trails, you will love Cambodia. Strongly recommended.