I'll be interested to see some of the responses you get for this thread. I am actually in the same boat as you. I was given a KitchenAid stand mixer in July and I picked up the grinder attachment for it. I have yet to use it myself. I can say I've had my butcher grind up tr-tip for me and it makes for a wonderful burger. Although where you're from Pennsylvania, tri–tip might not be easiest thing to get. it is almost unheard of around here.
Good luck with your new toy! Please post some pictures and tell us about your results. I'll be extra curious to see how you make out.
Tri-tip is also labeled as "bottom sirloin" in some markets. That might make it easier to find, although it may be sliced up rather than whole. If it's for burgers, though, that won't matter.
I don't have a KitchenAid mixer yet, but I might try to pick one up after Christmas when all the hyped-up foodie stuff goes on clearance.
Currently I use a manual sausage grinder and it works fairly well. I've also tried chuck, sirloin, and tri-tip in various proportions and they're all great. However, I've only used a couple of my rib-eye and striploin steaks for burgers. The last were good, but I'll be keeping them as steaks from now on - too precious to grind up.
Two other cuts I've tried are brisket (which dries out quick and is a bit tough) and flank steak (good in a mix if you're cooking it to medium-rare). But I've also parted with a few chuck-eye steaks (aka cross-rib steaks) and they make great burgers. Once again - too good for me to grind up, though.
One really odd ingredient I've heard mentioned from various relatives and a few chefs was trimmed beef heart. I've tried it and it really does bring out some of the flavor. I try to keep it under 25% in the mix, though, as too much of it gives the burger a sort of liver-y taste.