Pixel responsiveness is very close or as fast on latest IPS panels (with perhaps few exceptions naturally). And every manufacturer has its own definition of what response time in ms is. That is probably the most useless spec to look at.

Input Lag is far more important than response times. Here you can read more about it:

http://www.tftcentral.co.uk/articles/input_lag.htm

As I previously mentioned, there is a thing called "Video Overdrive" and every decent IPS panel has it. Some faster IPS panes don't have it since they don't need it. Even without Video Overdrive, input lag is in the worst case (full black to full white screen) a 1 frame lag (I am referring to the latest, and in this case the above two Dell monitors). IIRC this 23" model is equally fast as any TN panel out there.


The above two models, and the two HP's counterparts like ZR2440w & ZR24w (the one I have) will walk over any TN panel out there unless you need a 120Hz model for 3D gaming.



EDIT: Most people don't even notice the inherent horizontal color and brightness "shift" from normal viewing position on TN panels. But when they do, there is no going back.