I traded in an Audi A3 with the DSG for my Z, had several great years with the A3. Mine was even part of a "questionable" batch where Audi bumped the warranty up to 10yrs/100k miles after I bought it.
The A3 was great overall, but the DSG was simply amazing. Same unit that's in the TT, GTI, Tiguan, etc. I never experienced any shuddering, clunks, or hesitation with it, at any speeds. Around town, in "D" it acted just like a fast automatic; shifts were smooth, quick, and nearly imperceptible. Put it in "S", and it would downshift & revmatch with a vengeance, magically hold the proper gear through corners and bang off shifts so fast the tachometer could barely keep up. But never jerky, just unbelievably quick & smooth. Paddle shifters granted 100% manual control. Loved it.
I only experienced two oddities in the 45k miles I owned the car. Every now and then, on a slight incline in like a stop-and-go situation, if you were feathering the throttle, you'd feel/hear the DSG slipping the clutch a bit as the computer was trying to figure out whether you intended to idle or go. And three or four times, upon selecting reverse, it would not go into gear and just flash the "R" in the gear selection indicator. Flipping to neutral or park then back to R always fixed it.
The only drawback IMHO was maintenance. At least on the A3, the DSG requires a $400-$600 service every 35k miles.
So why doesn't Nissan have one in the Z? I think it's most likely a durability vs cost issue. VAG does the most with DSG's of any manufacturer, and in terms of VW & Audi, they don't put them in any application much over 200hp.
AFAIK, just the 2.0T & 2.0 TDI applications. While I suspect there's a comfortable cushion above 200hp, I bet that's about all the transmission is designed to handle. A4 2.0T's with 211hp|258 ft/lbs get CVT's, manuals, or a regular automatic. Lambo's and Ferrari's have DSG's, but that's obviously in an entirely different league of transmission, as the GT-R's is. I've read that they're working on a DSG for the R8; still a supercar. Ford's just now coming out with their first in the new Focus - again, a relatively low-hp/torque application. And I've heard not-so-great things about theirs from the media.
The GT-R's is Nissan's first attempt at one. Assuming it "works out" in terms of development costs and durability, it's likely that they'll take the lessons they've learned and apply it to future cars. Based on my experience with my A3, I'd buy a Z with a DSG without hesitation if it worked as VAG's did. It would make an incredible car even better. I very much enjoy shifting my Z myself, but that DSG really was about the best of both worlds; connectedness & control of a manual with the convenience of an automatic.
Though Nissan's focus seems to be more on CVT's (Maxima, Altima, etc). Imagine a Z with a CVT.