I recently upgraded to Lightroom 4, I had not downloaded the beta so I was keen to give it a try. After playing around with it for a bit, my opinion is that it's Lightroom improved. It will still be an essential piece of my kit. That being said it be nice if overall performance could be improved, especially got a major new reelase. I'm on a HP 8440w laptop (core i7 M620 at 2.67ghz with 8gb or ram) running Windows 7 Pro 64-bit.
Below is the same photo, one treated in Lightroom 3.6 and the other after updating the image processing to 2012 in Lightroom 4.
It seems I could control the shadows better in LR4 and that by just updating the image processing the photo got brighter and more contrasty. It makes me hesitant to go back to old photos and revise them as I would need to take the time and really tweak my adjustments. I still need to spend more time with LR4 to get familiar with all the changes but so far, so good.