I use LR4 and CS5 - I use CS5 for most of my processing although I am trying to use LR more to check out what it is really capable of. Plenty of youtube vids show that it can be a powerful tool. (unless you need to use layers of course)

For me the massive benefit of LR is in the catalogue functionality. It really is very easy to import, tag, catalogue, keyword etc in LR. It has made my library system so much cleaner and easier that I would recommend it just on that level without the added ability to apply adjustments to photographs.

On that, do we really need CS for the vast majority of our photos? Probably not, LR is more than capable unless we want to get really funky - and there are a number of add onss to help with that.

I get lots of fun out of using photoshop so I do not think it will drop out of my workflow - it is a part of my hobby - but I can see why you would want to use LR either standalone or as a compliment to photoshop.