I've not found it that beneficial with the XL2420t compared to some of the other 120Hz panels - does seem to tidy up some of the overdrive artifacts (mostly RTC errors) a bit but its already so responsive I'm not sure its worth the decrease in color saturation.
EDIT: Just seen this comment in PCM2's article on the RTS mode "Blue looks lilac and some severe oversaturation elsewhere. Generally looks messy on the desktop but some fantasy-based real time strategy could possibly benefit." sums up what mine looks like pretty much regardless of calibration or settings even with the best possible calibration that comment would still be true.