I just switched to a S600 (with replacement mouthpiece) from a G250, which I now use as my Octo. Yesterday I dove Molokini, and switched back and forth a bit.
At the surface, not much difference. At 90 feet, the S600 felt a lot smoother. I don't notice it cycling. When swimming upside down the S600 is almost normal, while the 250 takes some effort.
Main difference: I just don't notice the 600 at all, whereas I'm aware of every breath with the 250 and other regs I've used. Partly it's because it's light and small and doesn't pull on my mouth, and it just breathes really right.
Aloha,
Jonathan Starr
kalepa@maui.net