Another vote for "not an either/or."
Lightroom 1 could be described as having an emphasis on the "Light" part.
Lightroom 2.x was a whole lot better.
Lightroom 3 is a whole different animal - VERY powerful disguised behind a simple looking UI.
I highly recommend spending the money on the LL Tutorials for LR3. Do that, then download the trial version so that you may "play along" in step w/ the tutorials with your own files in LR.
Lightroom is very powerful in the data asset management department as well. Probably 90% of what I need to do in advanced image editing I now do in LR. The processing engine in LR3 is very good, the sharpening and noise reduction are now on par with most 3rd party products for most of what you'd normally need to do. And, in my book, the print utility alone is worth the price of admission. Very powerful, and very easy to use with the ability to create presets for favorite papers, ICC profiles, paper sizes, printer feed preferences, and a whole host of other parameters that can now be accessed with a simple mouse click. Future versions will almost certainly add soft proofing, and then it will be a darn near perfect printing utility.
LR's integration with PS CS5 is wonderful. "Round tripping" from LR to CS5 and back is painless. CS5 is still very necessary for what I call "pixel level editing" which consists of significant image manipulation of elements in the image, removing things, adding things, retouching, etc. It is also necessary for serious layers work (though the power of LR has diminished the value of using a zillion different adjustment layers in my work flow - once the real power of LR is understood, you'll realize how much CAN be done there).
So as of now, IMO, serious PP requires for me "both programs" in order to have a work flow that makes sense, is logical and fast and easy to use - yet can still get in there and do serious magic when necessary.