Some users — but not all users — have been having issues with Lightroom 4. Victoria Bampton, a.k.a. The Lightroom Queen, has written an excellent post on known Lightroom 4 issues, so I will refer you to it.
Should you upgrade despite these issues, or wait? I would suggest that it depends. Read through the list and see if these are things that would relate to you — for example, there are issues with the point curve (in the Tone Curve panel) and with plugins, but many users have never used the point curve and don’t use plug-ins. As far as performance lags, is time of the essence to you? Would you be willing to go back to Lightroom 3 if you experience unacceptable lags?
Rest assured, Adobe is working very hard to resolve these issues and release an update as soon as possible.
Finally, I agree very much with Victoria’s last paragraph — when you do update to Lightroom 4, don’t update all your photos to the new process version at once — do it selectively, as needed. (There is no need to update if you are done working on them or don’t need access to the new Basic panel or adjustment brush controls.) The translation from old to new can produce significant changes in your photos. This is not a bug, just the price of moving to different and much more powerful controls.
Some people report that converting just one photo to the dng format improves their lightroom 4 performance.
It seems that people with the high powered i7 machines are having more problems than people with lessor machines. I have a simple 2.1 ghz duo core pentium computer, Windows 7, and I find the performance of Lightroom 4 about the same as Lightroom 3.6.