If you shoot a large number of images. Need to file them and more important find them again Lightroom is the answer. I much prefer it's modular approach, and the develop module is much quicker to work with than the ACR module within Photoshop, although it's the same engine.
Photoshop scores if you have to do any pixel editing. You can't really clone , produce panoramas, or HDR in Lightroom. Also there is no soft proofing option within Lightroom. ( Although there are rumours that the shipping version of Lightroom 3 may have something along these lines, but it may just be wishful thinking.
Don't forget Adobe offers 30 day trials from their web site. Download and find out for yourself. There are plenty of tutorials on the web relating to Lightroom so check them out before your trial starts to get the most out of the trial.
Lightroom Ver 2.7 is the current working release, however Lightroom 3 is available in beta form to evaluate. If you opt for the LR3 option be careful not to commit to much to it as Adobe state that it may not be possible to transfer data from the Beta version to the fully featured one. The Beta is for evaluation and feedback.