After quite a while looking, I finally picked up a Millermatic 250 Mig welder at an auction the other day. This will replace a Harbor Freight/Chicago Electric "175" (in quotes) Amp Mig welder I've had for a number of years. The HF unit was a piece of junk. As delivered, the wire feed drive wheel was oval, which really didn't help the welds at all. I ended up reengineering the wire feed entirely, but that still left one problem: the duty cycle. At the high end, the duty cycle is about 10% (1 minute out of ten), otherwise the transformer starts toasting and the thermal overload pops. I've been building a Caddigger and welding a lot of plate from 1/4 to 1/2 and this was getting annoying, thus the Millermatic.
I had the Miller apart for cleaning and correction of a few minor problems, and had a chance to compare the units. I think the duty cycle for 250A on this unit is 40%. The transformer looks like it uses 1/4 inch square copper, compared to 10 or 12 ga on the HF. The voltage control uses a couple of SCRs feeding into a cap/inductor/cap filter (60,000 uf on each bank). The HF unit uses a tapped transformer, some automotive alternator diodes and an inductor. I can pick up the HF unit with some difficulty, the Miller weighs about 250 pounds. The Miller also has a thermostaticly controlled fan.
I had reengineered the HF unit to use a Tweeco gun, the orginal was pretty bad. The Miller had a Tweeco as well so I could swap guns (the Miller one was pretty beat up). The HF has a spot weld timer (and I kept that when I reengineered the drive), the Miller doesn't but that was a plug in option. I never used that option anyway. I added a gas purge and wire feed only switch on the HF which were pretty handy. I hate wasting gas when loading wire, or having wire run out when purging the lines. Those should be easy to add, though.
I mercifully do not remember what I paid for the HF unit, but it was probably more than what I paid for the Miller, particularly after rebuilding the wire feed unit. The cheap units are OK for light work, I suppose, but there's no replacement for quality. At least with the Miller, if I screw up a weld, I know it's me and not the equipment :-)