Mainly the sketchy charging handle is my biggest complaint about the actual gun itself, i'm pretty happy with my MPX. I didn't care about caliber conversions as I planned to shoot 9mm exclusively, but I really wish they would have had all options available at launch or soon after.
When I purchased my plan was to swap out for the 4.5 barrel and run a collapsible brace or stock, and have a great little backpack gun. Now the only option is to chop mine, or spend a crap ton of money on an SBR and wait months for it to clear to get the 4.5 barrel. Their lack of follow through and huge lack of parts really sucks, and just leaves me with a bad taste for Sig. I don't have any of their other weapons, and it will likely stay that way. From what I hear of their track record they are big on releasing "groundbreaking" new products and then no longer supporting them once launched.