There's a major difference between CS3 and LR. CS3 is both a Pixel Processor and Pixel Editor while LR is just a Pixel Processor. What this means is that if your main focus (pun) is on processing RAW images then LR will provide a very efficient way of working. A combination of CS3's ACR and Bridge can do a similar task.
If on the other hand you need to get creative and start editing the pixels then LR cannot do this for you. You would need to go to CS3 or Elements for example. You can do this from within LR, so that edited versions are retained in the LR Library.
Personally I find the Organising features in LR a bit lacking so currently I'm duplicating my databases. All Raw images are imported and processed in LR and then a TIF version is exported and picked up by Elements Organiser.
Similarly I find printing from LR a bit lacking - specifically the control over print sharpening and image re-sizing - so I do this from Photoshop.
Web galleries in LR are nice if you're into that sort of thing.
Slideshows are basic and I prefer Proshow Gold
Just my 2p worth
Colin