For the most part, Mac OS X has been solid and reliable. However, there are times when I have to restart. Recently my machine wouldn’t always wake from sleep requiring me to power cycle it; I don’t shutdown my machine all that often, but opt to sleep it so it wakes faster. I saw this a lot under 10.4.8 and it also seemed to occur when I unplugged my machine from my external monitor, keyboard, etc. With 10.4.9, I’m quite happy to say that I haven’t seen any of that. Some apps have crashed, but I haven’t had to restart my machine.
Way to go, Apple! Hopefully this will keep up with Leopard, but there are supposed to be lots of changes, that we might be starting over with stability.