oh christ yeah, to say there's a gulf in quality between ASG and Marui is an understatement.
That said, the ASG Mk23 is a solid performer, it's a non-blow back gun so there's not really much to go wrong. Direct clone of the TM dimensionally, different materials used obviously but TM mags fit and feed fine, though you may as well just use the ASG ones since there's no performance gain to be had from using the more expensive marui ones.