I've only noticed minimal changes. It seems slightly faster (subjective until I have time to use it at length in multiple situations) I do like the addition of Google Now but have found limited time to play around with it so I don't have any "cards"yet.
Project Butter seems to have all but removed the various ANR problems I've seen which is a big plus. Although I can't help but feel more RAM in the transformer line-up might help make the experience even more seamless.
Battery life is marginally improved, but my experience with that may differ from your own as I have the dock (rarely do i undock it) as well and cannot ascertain if the battery optimization affected the device itself, the dock or a combination of two.
WiFi and BT signal strength seem stronger and do not drop out as quickly at fringe coverage (but this may be due to network reconfiguration at the various locations I use it so I can't make a call on it)
I would say overall I am content at the moment, but really JB is a minor upgrade from ICS(as expected), and considering the TF300T has gone through several minor software updates and patches since its initial release, the JB upgrade at first glance doesn't seem too astonishingly different other than the items mentioned above.
I am sure that you may find a lot of google services/apps updates nice. For myself, I've updated apks for JB enhanced Google apps as soon as they were available so again I didn't notice as many changes as other may have.