There was a minor bug in the HiTechnic compass block that caused the comparision plug to output faulty values (the Absolute and Relative plugs worked just fine; it's just the logical output that had a glitch). HiTechnic has put out an updated version of their NXT-G Compass block, so if you've been using their previous release, please download and install the new one. Note that the Import/Export tool in the NXT-G environment will not remove a block from NXT-G: you have to do this manually. The HiTechnic download has very good, detailed instructions in the ReadMe file that comes with it on how to uninstall a block from NXT-G. Also note that when you install a new block like this, the Import tool may notify you that there are old version of some files you might overwrite (some of the image files are hard to find and remove); that's OK, just go ahead and install it.
HiTechnic homepage (go to the downloads page for the updated 1.1 block)