I have had every version of Lightroom since it was released but I didn't start using it until last year. It's an important part of my workflow and it looks like LR4 is a keeper... but I'm still digging.
Along with the usual sliders (Temperature, Tint, Exposure, Contrast, Clarity, Vibrance, and Saturation) they've added/changed to Highlights, Shadows, Whites, and Blacks. These controls add a lot more flexibility to the image editing process imho. The clarity slider works a lot better than it used to. The book making tab and the way the print engine works (lighten and contrast adjustment that you make specific to your printer for instance) seems to be a lot better too. I think this one is going to be a winner.
LR4 is a little buggy (it's a beta version so that's to be expected). For instance, you can't use the cool new tools on Raw files. I got a warning that I needed to update to ACR 6.0 but when I checked for updates in Bridge and CS5.1 the only version of ACR that is available is 5.5. I'm assuming that there will be a 6.0 out in the near term and it will have the nifty features of LR4. Continuing along that line of thought I'm assuming that Lightroom uses the same raw processing engine as CS. which makes perfect sense to me. Either way, for LR users this stands to be a really good upgrade.
I will do the same. I wanted to see what was up with the new release. I could have just watched Scott Kelby and Matt Koskowski's video's on the NAPP site but I wanted to see it for myself