BTW, for all of those who think a sentry has to be the engineer's primary weapon, look at my weapon usage stats for the last week! 276 kills with shotty, 119 with lvl 3 sentry, 101 with level 1 sentry, and 69 with pistol. I usually will use a sentry to distract the opponent and keep them from attacking me while I wail on them with the shotty! They have a harder time shooting me while the sentry is shooting them, and (especially with lvl 1 sentries) they have a harder time finding the sentry when I am shooting them.
I have gotten ludicrous kills to death ratios (one of them was 30 something to 1) while doing this, and it gives my team a nice safe area to retreat to if they are being pressed!