I just upgraded from the FX-888D to the FX-951 and could not be happier. The "sleep" option is great and the tip is back up to operating temperature by the time I am actually soldering. It's much easier to change the set temperature, as it has actual up/down buttons.
It also seems to maintain more consistent tip temperature vs the 888D. I zeroed the temp offset calibration and with a little solder wetting of my thermocouple tip, I measured 680.1F with a set temp of 680F. Haven't tried measuring the temperature during actual soldering, but I am certainly better able to maintain consistency with this iron vs the 888D.
The integration of the tip and heating element does increase the cost of the tips, but I like the benefits. It's much faster to swap tips, and the include a cute tip holder with the 951.
I still have my 888D as a backup, and for cases when I need two irons at the same time (happens periodically in SMD work, especially when desoldering).
Just thought I'd mention the option.