It kinda depends on how you shoot and edit. If you edit in Adobe Camera Raw (which comes with CS3), then you probably don't need Lightroom since they are very similar. Either one is easier than editing using adjustment layers in PS, though, since you have a menu of "sliders" that lets you adjust almost everything about your photo (white balance, exposure, saturation, etc) all in one place rather than having to open a whole bunch of adjustment layers (like Curves, Hue/Saturation etc.).
Don't know if this helps, but here's some info from Scott Kelby, photoshop guru:
http://www.scottkelby.com/blog/2008/archives/1308