Evolutionary, not revolutionary
I am pumping for my 2nd baby and previously owned Freemie cups for pumping discreetly. I would say that of the two, I prefer the Flow, but it is not a revolutionary product.
Flow has fixed a few gripes I had with Freemie: the hanging bottles make milk storage cleaner, easier, and more discreet. Milk also does not, as a part of the product design, get into the air path when you empty out the milk, making it theoretically possible to stash Flow in a refrigerator and reuse it again. The Flow also has a slightly lower profile, making it fit under clothing that the Freemies will not fit under, and has a nice clear line of sight to your nipple so you can figure out if it's working.
However - Flow is not what I'd call super discreet. You need to carefully select your clothing so that it is not obvious. Definitely don't wear knits, for instance. It doesn't fit under a standard bra; you need to use something with a stretchy band. Of the two stretchy bras I tried, neither was particularly good at keeping Flow in place, but thankfully the suction created by pumping prevented it from falling out altogether. The Flow also kept shifting around; which didn't affect the pumping, but was distracting and looked odd. I'm tempted to start using it with a pumping bra.
Flow is better than any other product I have tried for pumping discreetly on the go, and the price point makes it a reasonable splurge for traveling moms. I pair it with a Spectra 9+ and have successfully pumped in public. It is not however a revolution.