I spent a bit of time getting my 200 Amp Mig running aluminium and I'm getting good results now. mickmate is right, you get your best results in spray transfer. That means lots of current and wire and a steady hand cause you've got to work quite fast. I've been welding thicker materials with mine and getting quite good at it. Preheating larger pieces certaily helps. I've got a portable gas stove that I sit the larger work piece on. I welded up a manifold for Bernies Cobra a while back and the gas stove worked a treat for preheating it.
Getting the teflon liner, rollers and tip right is the key I reckon. I went through 2 liners getting mine right. You need to set up the liner so it so close to the rollers that it rubs on them. This stops the wire from bunching up at the rollers if your arc goes out and the wire hits the work piece or if it sticks in the tip. You also need the brass sleeve that slides over the outside of the liner to keep it straight where it meets the roller.
U grove rollers have more surface contact with the wire and mean you don't use so much tension on the pinch roller. The wire will still drive smoothly but you can back the tension off enough that the rollers slip if the wire jams. It takes a bit of experimentation to get the pinch roller tension right.
The other trap is the tips. 0.9mm aluminium wire tips have a larger hole than a 0.9mm steel wire tip. Aluminium expands more when heated so it needs a bit more clearance or it'll jam in the tip. If you don't have any aluminium tips available just use the next size up steel tip i.e. a 1.0mm or even 1.2mm steel tip for 0.9mm aluminium wire.
You can get some great results on nice clean billet aluminium but castings can have variable results due to the porosity of the material and casting sand trapped in the pores. The weld may look good on the outside but hit it with a grinder and you may see bubbles.
Here's one of my home made quick jacks prior to polishing. They were water cut from 6mm aluminium plate.
Cheers
Michael