Our 401A and 500A were both problematic when we got them. As I've used them I've de-bugged them. They are now two of my favorite machines.
My wife and I used to argue ( in fun ) over the 401, now I tell her the 401 is hers and the 500 is mine. No arguing, no fun either.
The only time I have had trouble with the 500 is with the stupid splices in the C&C thread or I threaded it wrong. Otherwise it's pretty non-picky.
Joe