I am going to get a lot of crap for this, but honestly only having the 9400 I do not feel like I am missing much. I was worried about this as well when I was getting my replacement computer from Apple to replace that blasted faulty 8600 that died on my machine more than two times.
First of all, I do game, whether it's on the Mac side or Windows 7, I play 3D games. These games have ranged from Portal, America's Army 3 and Combat Arms all while also using Aero graphics. I have not noticed any stutter performance on Windows playing games or performance wise with just the 9400.
Actually I notice that the 9400 runs cooler than what the 8600 did. And with now visible side effects, really makes the card a plus. I know I will probably be flamed by some 9600 lovers though who will argue their microseconds differences in benchmarks, but for me being on the consumer and pro side of the argument, the card is better than most dedicated solutions I have seen in the past and really blew me away. I was worried I would see a performance decrease with all I did when I 'upgraded' to the newer system.
On the Mac side, animations are still very fluid and the only time I get a stutter I usually when an application goes rogue and memory leaks and hogs the system's resources. I have played COD4 on the mac side, Guitar Hero 3, Half Life 2 through a Wine-like wrapper and have not experienced any slowdown with the graphics in mid-high settings. In fact, most of those games, are ports of Windows games that are wrapped in something else and in theory still run smooth even though they are not really running natively on the Mac.
Though games, say 2-3 years down the road I would expect to start having issues with graphics in the mid range and possibly on the lower settings. You really can't expect any card though to have a long self-life though now-a-days and in 3-4 years I'll probably be pricing a new Mac anyhow so I don't really expect to see that day when this card can no longer handle basic gaming.
I also had a demo of Maya on here for a project, Adobe CS4 and Final Cut Pro and have not experienced any major set backs in any of those applications with what I use them for. If you would try to render a scene from say the Transformers movie out from a program like Maya, I would say that the 9600 would definitely finish faster by a noticeable difference.
I am a Digital Media major in school. I only really experienced some hiccups in Maya. I had some issues in Photoshop for the first time the other day while editing four 24+ mb pictures at the same time. Photoshop's memory in Activity Monitor was registering at 1,000 +.
I may see a tad difference when I plug in my 20" external monitor as a desktop solution in a few weeks and change the resolution from the 1400x900 now to something larger. I will be shutting the lid and using the external as the primary monitor when I am at my desk at home. But I am not sure if I will really see any change there. Guess time will tell.