I think it depends on what you want to do with your images.
I only use Lightroom 4 currently because I try and simulate what is achievable in a dark room. That means, like Damion, I'm only adjusting brightness & contrast (by whatever method), dodging and burning, spotting, vignetting, outputting to Flickr with a border, etc. I also find Lightroom so much easier as I am not dealing with a multitude of files, only the one file from the scanner.
The only reason I'd say you need to start getting into photoshop is if you are masking, combining images or anything more complicated than that.
Edit: Currently Lightroom 5 is on offer in the UK at 25% off, not sure if that offer applies in the US too.