I just installed 1.5b1 at a WordPress 3.4RC1 test site that has 25,000 complex test pages and a wide assortment of other plugins installed. It seemed to function correctly until I optimized JavaScript code with YUI Compression using a customized version of the Autoptimize plugin, which caused web pages to no longer load. Of course, that doesn't necessarily indicate anything is wrong with 1.5b1, but merely that there is an incompatibility with that combination.
I haven't done any debugging yet to learn why that happens. I will do some troubleshooting and let you know what I find.