My players like the 4 color Desjgn. They are pretty, and they make flushes unmistakeable, which I consider a good thing.
The colorblindness thing as discussed earlier seems a real concern. None of my players (so far) are colorblind, but apparently something like 8% of men have some significant difficulty distinguishing red from green.
In 2 color decks, clubs are the same color as spades, and hearts are the same color as diamonds. This is not a big deal because everyone is used to it. But in 4 color decks, clubs are typically green and hearts are red, so for a red-green colorblind person, hearts are now the same color as clubs. This is weird.
I'd like to see someone try yellow instead of green for clubs.