Here' my 2 cents on the subject:
Some years back when my shop was busy as heck, we subbed out a job to a "pro" welder guy to put in some 7GA sheet steel on a tuck body - he worked out in the lot out of a worktruck with gas driven mig. He used flux core wire 'cause he was working outside (the reason he gave us, anyway) and I must say we where disappointed in the look of it when he was finished. It wasn't a big deal, structurally it was good work, but it was noticable that the beads didn't look as nice - had spatter all over them - would'nt matter if it hadn't been all over the sides of the truck and made it obviuos what was repaired even after a full repaint. I wished we'd had the time for me to just do it myself and have it look as good as factory work.
I also have a spoolgun for aluminum (35A maybe - can't remember exactly)- works great, but you don't have the same control of heat like you would with Tig - I find you have to startoff slow 'till the work gets hot and then make sure you stop before it gets too hot and blows right through (thats harder to judge on aluminum, too). On thick stuff, I go so far as to preheat the work with torch to even things out - other wise you get and obviuos "cold" bead laying on top of the work, or else a giant puddle at that end till it's hot enough to really burn in. Also, at the end of a bead, hold the nozzle at the same spot you would while welding, don't just let off the trigger completely and pull the gun away. Release the trigger enough to stop the wire, but still in far enough to blow gas to allow the argon to sheild the bead 'till it cools - keeps the air from contaminating the bead / making it ugly. From what I have experienced, the spool gun will get the job done to put some aluminum together, but I don't think it's possible to get the perfect "row of nickels" look form Mig'n AL - that takes Tig (but maybe there some expert out there better than me!).
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