Playing as a guest, I often wished for a "pause" button; many games recorded a lot more time than I devoted to the puzzle. However, a pause button will skew the stats since there are players who will pause, solve, "continue", then pop in the answers. Perhaps a quit button (without a surrender) and the game would not be recorded in stats; also, if game timer is running for hours, do an automatic quit on it, perhaps asking first in case a player is really spending 2-3 hours on one puzzle. The timer is running now for a puzzle that was giving me fits; it will probably put me in a statistical hole from which I can't recover.
I have another suggestion re grid/hints; I'll try to find a more appropriate place to put it.