I've used a spool. It's better on 1/8" and up. It'll burn holes in the thinner stuff. Otherwise, if you clean the area you want to mate, it works pretty well. That means removing the oxidation. If you don't, welding will be a mess.
I had a Miller set up for welding aluminum only. But before I could play with it, it had to leave.
On my current Miller with the spool, it is already set up for that equipment. A place for the tank, connectors for the leads, etc. To adjust the settings for welding aluminum, you turn on your gas of choice and trigger the spool. The machine will read that you are in aluminum mode and will allow you to adjust at the spool gun itself. To go back to the MIG gun, trigger that one and it will switch back to the steel wire feed controls. This machine also comes with a bit of a cheat sheet. If you look at the upper part of the picture, you can see that they give you some basic settings for the particular type and thickness of metal that you are welding. Good starting points.
