the only time I think seriously on the SAG is when I'm facing MEQ or 2+ save armies. The template is ap 2! so regardless of what strength, it ignores most armor. Having said that, grab yourself all three ammo grots because odds are, you will not hit your target(or at least that's my luck) and the ammo grots are worth thier weight in gold. I like to put him in a squad of boyz instead of grots. Mostly because my SAG tends to port him and his squad into assault way to often. Remember, you will live and die by the SAG. He's either a huge point sync that has done nothing to help, or .....the piece that won the game.
As for the KFF mek. He is by far the most hated piece in my army. He always comes in handy. Marching a green tide across the field, give them a 5+ and watch how many more make it across. Remember, that if a squad is in front of another squad the back squad gets a 4+ cover save, so as long as you position your green wall right, the front should be 5+ and the back 4+. Also, I throw him in a battlewagon, regardless of what squad I'm putting in thier. It's 6" from the edge of the battlewagon. Gives all my other vehicles a 4+ cover. Nothing makes you feel better than watching that drop podded stern guard unleash a salvo of melta fire at the rear armor of my battle wagon, only to have it survive thanks to the KFF, and turn to fire it's killkannon the next turn wiping that squad.
I rarely run the SAG, sometimes only because I get the need, like gork (or mork) is telling me to field it. It's lost me more games than won, but don't get me wrong, every game I played with it was entertaining. The KFF is the juggernaut though. It makes the frail into the frightening. I've fielded one in almost every list, even forgoing a warboss on those occasions where I want a SAG.