I can only really speak from a personal perspective, not really being an experienced PS player like others on here. I recently upgraded from a pretty basic GFI Student E9 to a Carter Uni 12 which I bought from John Roche, thanks John

I fancied the U12 option as it seemed to offer the most opportunity for expansion of future development and experimentation. I guess being relatively new to PS I'm looking at this from a different perspective to established gigging players who cut their teeth on E9 and C6 years ago. I'm not looking at it as E9/B6 as such but more of a different unique tuning altogether.
I'm aware that the basic elements of the Uni tuning come from E9 / C6 and I will be looking at tutorials for both tunings to learn the traditional uses of both tunings, but I would like to eventually incorporate these into one style of playing.
I guess it depends on what you want to get out of it, I'm getting on a bit now and don't really see myself doing much gigging in the future, though I still like to get out and play live every now and then. So for me it's a bit of a musical adventure, one which I'm enjoying immensely and the U12 opens up so much opportunity for experimentation. I have a small studio in the house and like to muck around in there for hours, my wife says that it is like the Tardis, time flows at a different rate in "that" room

I decided it would be a good idea to buy a Uni now before my muscle memory got too stuck in it's ways! and it would be difficult to unlearn things, such as the knees being the other way around and the loss of the D at string 9. Strangely enough I've got used to these changes quite quickly.
Different strokes for different folks I guess.
PS I really like the sound and resonance of the low strings

PPS Just watched Slimbo's video, sounds really nice
