The Flex 3401 is at the top of its game, largely a result of it being the only one in its category - forced rotation DA. Even if it wasn't the only one in its category, it's still a durable workhorse that puts up results and does it all, compounding and finishing to perfection.
Me personally, I wasn't sold on the Flex because I already had a DA and rotary combination that was working for me, and the price of the Flex is a bit too high for me to make any switch. Although the feedback from the 3401 is more manageable than other basic DA's at high speed, on my hands the rotary is smoother doing extended compounding, so any two or three-step project would still have me reaching for the rotary. Not to say you'll have the same problem, my hands are in the early stages of arthritis.
Both the rotary and the Rupes are alternatives to the Flex. For me, each one has its individual pros and cons that make choosing between them a lot to think about, not to mention price. Like the rotary, there's a learning curve to the 3401, it's going to feel different on the paint than a regular DA due to the forced rotation, only compared to the rotary the Flex rotates in the opposite direction as well as orbits. Safe to learn on due to that orbital feature, but those not expecting that different feel may be turned away from it without giving it a chance for them to get used to. Some guys may lean towards the Rupes for its familiar feel to other DAs, and also its balance and incredible smoothness. But in some curves where the Rupes has a tendency for its rotation to be stopped, the Flex will power through those areas without adjustment.
You just have to match your preferences to the price you want to pay.
