I've upgraded from LR2 to LR4, so it was a pretty big change for me. I really like the map module. I had been using a plugin to do some of the same stuff that the map module does, but this is much nicer. I'm still trying to get use to the new basic sliders in the develop module, but I do like the results. It just may take me awhile to readjust my workflow. My only real complaint is that it seems much slower than my old version. This may be because the program is starting to max out my processor, though I'm not sure. I'm hoping the soon to come update will improve the speed performance.
I have upgraded from LR3 to LR4, and have not regretted it a single moment. LR4 has a new processing version which gives outstandig possibilities for tweaking tones, far more better recovery of the shadows then in any other version, and also the noise reduction is better in my opinion. Very intresting is also the new book module integrating with Blurb, which I intend to use shortly. I use LR in combo with Nik plugins and enjoy that a lot. LR4 has got softproofing now in the print module which is also great !. Do not need Photoshop, have PSElements, and really only use that for some content aware filling.