I tried the Opti Navi Key for a good 10 minutes at a priority dealer. It sucks. Every flick of the thumb takes you across only one icon in a menu, and the sensitivity option doesn't change this! So while the first 10 seconds on a youtube video may fool you into thinking "wow, what a cool new feature!" IMHO this in the real world is absolutely useless (i.e. the particular way the e72 has it). Just using the old D-pad buttons is a whole lot more efficient and pleasant.
Now if you thought that someone (Nokia or developer community) even fixes this with a firmware upgrade, there's another problem - the physical design of the button. The opti navi key sits sunken into the regular d-pad (like on the e1, e51 etc.). So when you flick your thumb on it, each time your thumb is hitting the raised edges of the D-pad. This gets very annoying and very unpleasant very quick. After 5-10 minutes, my thumb felt a little sore.
I compared this with the touchpad on the Blackberry 8520 which is miles ahead. One swipe (not flick, but a light swipe), takes you across several icons, which actually makes this an efficient process, unlike the e72. Unfortunately, the 8520 has it's own limitations (poor plastic build quality, no gps etc.), so neither phone is really worth the price IMHO.
I was really excited about the e72, but the opti navi key totally sucks. I have decided to hold off till next year for a better phone to come out.