I used the RedRock M2 adapter whilst at film school with a series of prime lenses from a 35mm SLR camera and thought it gave a really nice and soft film look. I used my Nikon 18-135mm zoom lens from my DSLR also but this gave it a more dark and even distorted look. Maybe someone with more experience could elaborate on this.
I think they have updated to the M3 model now and it is still quite expensive to buy - but could be a worthy investment depending on what sort of stuff you are shooting.
They thing I did like was the ability to blend footage from the RedRock and standard lens on the HVX200 together in one project - fell free to check out a short film I made, where I shot different scenes using both:
I have the HVX200 with the Letus 35 Extreme/Nikon lense mount. I have 3 primes and a telephoto.. It too works well and adds a soft grain look.
You can also mix stock lense footage with adapted lense footage. I would make sure to check your adapter package THOROUGHLY as soon as you open the box. I got mine from B&H as a total production package... but i could hear the ground glass jiggling inside- they use some sort of cheap glue/adhesive.. needless to say it took about a month of "customer service" and "let me get you over to the manufacturer/distributor for further assistance". I missed several test shooting days that resulted in not being able to use the lenses on a short film.