The main difference between the ZO2 and the Nitrox is that the ZO2 can be used with the Oxy2 O2 sensors to monitor the O2 in the breathing loop of a semi-closed rebreather. It's designed for the Dräger Dolphin, but adapters for either the Dräger Ray or OMG Azimuth are available. It can also be used with the pressure transmitter for OC just as the Nitrox. As mentioned before, the ZO2 can calculate higher O2 mixes, but it doesn't have a dive planner.
I like the display, after the HS Explorer I think it's a close second. No screens to go through, all the important info is there and easy to read. The breathing rate warning can be adjusted (or switched off), but it's a good tool to become aware of when and how much air your sucking. It's wrist mounted, my preference. Good backlight, good protective cover, good graphing once downloaded. Did I mention, great display?
What I don't like: doesn't do RGBM or microbubbles in any way (that's what i call obsolete), wet contacts are a pita, memo mouse is a pita. Memory is ridiculous. It'll memorize 19 dives, but graph only the first five (if memory serves me right). Hence daily memo mouse downloads (mixed bag, it's smaller than hauling your labtop). Most parameters can only be adjusted by computer, much info only be retrieved by it. Battery change can't be done by user. Uwatec will test the unit after the cahnge, but the thing will take time and money (postage/insurance). If violated goes into lockout and sends 'info package' for computer download. No way to access data without PC.
The latter by itself is reason for me not to use it in the first place. Unless using one of the above mentioned RBs, it's a no-go for me.