Wow! I get asked today's question every time I teach a class, do a program for a camera club or have discussions with people interested in getting recommendations about which programs to use. The reason I've qualified the question to only Adobe Photoshop Lightroom (3) versus Adobe Bridge CS5 is because of the database functions. The Develop Module in LR3 and Adobe Camera Raw (ACR) are the same thing. So, if you're interested in the Develop Module and already have either Photoshop or Adobe Photoshop Elements 9 (or now PSE 10), you have the Develop Module of LR3. They're the same program. The interface is slightly different, but layout does make the program. The ability to do things is the issue. So, my recommendation is always to stay with what you have if the Develop Module is the only draw for going to LR3.
The database portion of LR3 is far superior to the current versions of Bridge. One demo I do when giving a presentation is to make a Smart Collection using Bridge. I push the keystroke to get it started and then open LR3. In LR3 I make the exact same Smart Collection. I press the key to get it going and almost instantly the Smart Collection pops up. I then switch back to Bridge, and it's still plugging away trying to build the Smart Collection. The speed at which you can find images is a huge plus in favor of LR3.
The general layout of LR3 is another plus. Where Bridge has everything hidden in menu after menu, LR3 has everything needed for the Library organized into panels. Start from the top and work your way down through each panel and you're done. Nothing has to be searched for. It's all right in front of you.
Keywording, ranking, rating, collecting images are very straight forward. If you want to keep track of your images, grab a copy of Lightroom. If you're interested in the Develop Module and have either CS5 or PSE9, stick with what you have.