I used them often enough. They're great with mortars!
............Except that mortars suck. They are nice to re-roll canon misfires, but, usually you want a Wizard instead. I'll be taking one in the 7th Edition, 'cuz a the bird bombs, mainly. They're S4 with unlimited range!
P.S. - It is nice to have a repeater handgun when your war machine gets charged. Almost ceratinly not worth a hero slot though. Now, I'll be fielding 3 Great Canons on Tuesday. I know I'm taking a big risk not having a Master Enginner in with 'em. But, I ain't usin' 3 hero slots either.