I'm kind of in the same boat as the OP. My B5.5 is nearing 140k miles and I'm not sure it ever had the fluid/filter changed. When I bought the car about 8 months ago, the Carfax showed 49 previous services, all at the VW dealer in Fargo, ND, with many regular 5k, 10k, 15k & 30k services. But, as you know, changing the ATF fluid is not part of the regularly scheduled maintenance, so I can only assume the fluid is original. I never drove one of these cars when they were new, so I have no idea what the tranny felt like when new. Mine downshifts like right now when you press the gas to order quick acceleration, but is somewhat sluggish on the upshifts unless you really gas it hard, and seems to be programmed to upshift early for fuel economy reasons. I have not noticed any slipping at all. Compared to the autobox in my Honda Element it's a rather crude transmission, though I will say the Passat tranny shifts down and hunts around a lot less here in hilly western N. Dakota than the Element when using the cruise control on the highway, but I attribute that mostly to the far superior aerodynamics of the Passat vs the blocky, upright body of the Element.
I spoke to my current VW service people about servicing the transmission and they seemed to lean toward NOT servicing it. For now I'm leaving well enough alone. Is there any way to inspect the fluid without servicing the transmission? I guess I haven't even looked in the engine bay to see if there's a diptstick for the transmission and just assumed there isn't one.