Well, I received the flex 3401 and a rupes 21 all in the same week.
I detailed a black denali this weekend and got to use both for the first time.
Let me say, based on forum talk, I was more eager to use the rupes tahn the flex. So much so that I ordered it before the flex.
Well, I noticed how smooth the rupes was, but quickly became frustrated to a dusting issue I had with the blue rupes pad and rupes compound. I did not have time to mess around, so I went for the flex for the first time. All I can say is WOW! I love this thing. It corrects better than the rupes imo. I used a 5" LC hybrid orange pad with menz fg400 and I was having fun. I love the fact that it's direct drive. It kind of makes me feel as if I know how fast it's correcting. In other words: it has great feedback. With the rupes, I kind of wondering what's going on when I am making passes. The feedback on the flex is outstanding. It's as if I can feel the amount of friction being produced between the pad and paint. I like that. With the rupes, it was more like gliding; I didn't like that. When the two units showed up at my door, I thought the flex looked like better quality. That's not to say the quality on the rupes is low--it's not. It's just the flex's is outstanding. I could go on and on about the flex. From my experience with the two, which is very little, I believe the flex will be my "go-to" polisher. I am thinking of buying another one, just to own two of them so that I don't have to use my pc for tighter areas.