It depends what you want to do. I know that sounds evasive, but no one machine is good for everything. Thunderbolts are an industry staple (just like the Lincoln 225 tombstone models). They are light duty, non-industrial machines. If the machine is an AC only model, it is a good general purpose unit that can weld anything from heavy gauge sheet all the way up to heavy plate. It takes a lot of practice to lay down good welds with an AC stick machine though -- that's the nature of AC current. It's not good for traditional body sheet metal work; you need a MIG or TIG to weld the thinner material. A basic AC unit will allow you to do gouging and cutting with special electrodes -- just don't expect the result to look like a plasma cutter.
If the machine is an AC/DC unit, then you have a much wider range of options as far as electrodes are concerned. DC provides a more stable arc and is easier to control. This flexibility will allows you to be more effcient in welding at odd angles and overhead positions. Since you have more electrode choices, it also allows you to weld steel and certain alloys more efficiently than traditional AC.
Most guys that own an AC/DC machine will set it to DC and forget it -- DC is much more efficient and verstatile than AC operation. That said, if you get proficient at controlling an AC arc, then there isn't much that the machine won't handle.
