Some irrational types such as myself prefer new. I'm just a home-gamer with a 44" US General but I can see myself making a poor financial decision to buy one of these in a few years.
I feel the days of cheap used toolboxes are coming to pass, as the general population becomes more driven to try to profit as much as possible ("I know what I have" mentality) and conversely, everybody wants a "you ****"-caliber bargain.
Personally, I feel that with my 52" Husky box, the only move that would make sense is to the US General 56" -- I don't have room for anything larger at the moment, and I could use one or two more semi-deep drawers.
I'm not sure where the Icon toolboxes will fit into Harbor Freight's lineup, just like I said about Icon torque wrenches.
There are definitely some items missing from the HF offerings though, and if we see a good no-skips, reversible, ratcheting, non-China combination wrench set I think it could be a winner. Throw a WrightGrip style open end on there, and I might be buying another set of wrenches just because.
I don't feel the US General boxes really lack much, then again I don't do professional automotive work, and would say I'm more of an enthusiast.