It's still too early to tell how widespread these problems are, but I have two reports of "hard freezes" (requiring a restart) after installing Kaleidoscope 1.8:
Dennis Dal Pra writes: "Whenever I clicked on an item in the Kaleidoscope control panel (Help button, font selector, scheme selector) my system froze. I reinstalled Kaleidoscope 1.7.3, which continues to work flawlessly."
Tony Martin writes: "When I click on the 'Send' button of NewsWatcher 2.2.0, the Mac freezes. When I disable Kaleidoscope, the problem goes away."
Update: Bob Thomas says that moving K'scope to the end of the loading order may fix these problems.