Interesting review, TTQ! I don't have any exposure to the Flex 3401, but I see your points regarding the Mille. For me, the biggest annoyance is the trigger lock button, that I am touching by accident while polishing, which of course shuts down the machine. Then the slow start feature kicks in, which compounds (see what I did there?) the annoyance of the interruption. It's not a deal-breaker, by any means, but it does take away a bit from my enjoyment of using the Mille. I'm using the Rupes Mille pads, and they work very well. The blue foam pads cut much less than you would expect them to when you feel how rough they are. The blue pads seem to break down if you get a lot of product and residue in them, so I started changing them out more frequently. The yellow pads flat-out rock, though. In fact, they finish out so well, that I haven't even tried the ultra fine polish and white pads yet.