I don't like speep mode. We had several cases of where the machine did not go into sleep mode the way it should and just sat in it's case closed with the fan and the rest still running. It gets very very hot when it's in a closed leather bag.

It was a known bios problem on the line of dells we were using at the time (well at the time it was not known, but they updated the bios to fix it). I prefer hibernate, and never put it away until it's shut down.