For me, the hardest part about working from home (been doing it since late 2006) is dealing with unproductive days. Some days, try as I might, I just can't seem to get anything done. I end up feeling guilty about this, far worse than I ever felt about wasted days while in an office. The fact that productive days will (hopefully!) outweigh the bad ones should be consolation—but in practice it's not much of one.