I have a 6-speed. I had to go all the way to North Carolina to find one in decent trim with a manual, that's how much I wanted it.
There's no question the cambio would be faster around a track or in a tight canyon road. The shifting is a bit faster, there's more control, you don't have to take your hands off the wheel, etc. etc. But the Maserati is car that's all about feel; the acres of steering feel, the snarl of the motor when it opens up, the road feel through the tight suspension, etc. etc. I wanted that extra level of feel and involvement that a manual would bring. This is why I hate the new 5-series--it's like a driving simulator from all reports. The Maserati feels like cars of yore. I'm happy with it. If I bought again I'd probably get a GS just because I've done the coupe-manual thing, but that doesn't mean I regret it. I love my car.
I was also a bit wary of buying a cambio given that it's a new technology and Maserati isn't exactly Honda when it comes to reliability. So the decision for me was obvious from the beginning.
That being said, the manual is not the best in the world. It's heavy (which I like, but others would criticize), first is a bit tough to engage when cold, and it's a noisy little fugger. It lets out a little grind when you redline 2nd and rush to 3rd, but goes in fine and according to my service advisor is normal and not bad for it. The shift boot has popped out once or twice when getting reverse. It does it's job fine and it doesn't detract from the driving experience--rather it gives it a more manly feel, heh. It takes some muscle to work that thing right.