No glitches, drama, instability or application incompatibility. Snow Leopard was probably the most boring upgrade I have ever done. I told my wife not to plan anything this weekend to give me time to tinker and fine-tune everything, but nothing was needed. I figured I'd at least be forced to clean install one machine, but there has been no need to. Love all the improvements too. Everything feels faster and more responsive and generally smoother. $50 well spent.