I have an 18 sierra denali 6.2 bought new in march with 6800 miles on it now, and it seems like I'm one of the lucky ones without the shudder based on the descriptions I am reading here. I do distinctly feel the grab and release of AFM/V4 mode but otherwise it seems relatively smooth. There are very specific circumstances at low speed when the trans seems jerky. Such as if I'm rolling to a stop sign from say 20-30mph and I slow down to maybe 10-5mph but not completely stopped, then I apply throttle to pull out and accelerate again. The trans jerks and its as though it isn't sure what gear to be in for a couple of seconds.
Also I park at the bottom of my driveway so every time I get in to leave I have to climb a bit of a hill, a couple of times it is like the trans is in neutral until I give it more throttle then suddenly it grabs and jerks and takes off abruptly until I let off the throttle. One time I got in and pulled it into D and gave it gas and nothing happened at all, it was completely in neutral with the engine free revving under throttle even though the dash indicator showed D. I put it back in P then back to D and it went normally.
Other than those two oddball issues, my trans seems okay. I would still like to change the fluid just as a preventative measure, even if that means I have to pay for it and do it myself.
I did inquire at the dealer about the possibility of evaluating it for a fluid flush for the known issues, and the service writer told me something like "we cant do anything, gm isnt sending us any fluid until spring" which sounded like BS.