If you have ever played or have ever wanted to play a thief character you need this book. Before I got this handbook, no one really wanted to play the thief in the group and usually was just a second character for someone. Thieves just open locks and occasionally fire their bows right?
WRONG. This book opens up many faucets of the class that I had never thought of and expands on the rest. The kits in this book really give character to your thief characters. Ever wonder why thugs were always thieves class, well check out the bandit or thug kits. I have heard complaints about the kits not having a lot benefits\hindrances (and the majority don't), but what they do offer is flair and adjusts abilities (including thief abilities as well as proficiencies) to better fit the thief's many roles.
This is also the only handbook that has any good new proficiencies. I especially liked alertness, fast talking, intimidation, and observation. I also liked the new equipment section. WOW! It has all kinds of cool stuff that would make any thief character's mouth water and don't even unbalance game play. The book is worth it just for this and the kits.
But there is more. The book also gives new info on lock picking (advanced locks, modifying checks) and on poisons and antidotes. There is a great deal of info on thieves' guilds. A LOT. Probably even more than you would want to know, but that's okay. Use what you like skip what you don't. If thieves' guilds even show up in your campaigns, I highly recommend this for DMs. The book also gives useful info on running thief based campaigns that gave me lots of ideas on how to make life in the city a little more interesting. Ever want to run a campaign solely in Greyhawk City or Waterdeep, well try it.
All in all this is a great resource for players and DMs alike.