The BIGGEST problem I have is the heat and the fan noise. It wouldn't burn my lap but it is so hot that the fan is running 24/7! Even when I am only surfing the web the fan is on and makes a considerable noise. The only time the fan is off is right after startup for about 5 minutes then it kicks in and goes and goes and goes. It's driving me nuts. After having an almost completely silent computer with the iBook this is just annoying.
Make sure the processor performance in sys prefs -> options is set to AUTOMATIC. My 15" pb had the fan on constantly because I had it set on highest. It worked for me.