The Neptune has been a great altimeter for me so far. The altitude read out is always accurate when cross checking with other peoples altimeters. When suited with a rubber protector it is very durable.
However, in my experience the internal logbook has been very inaccurate. I have seen my log function say that I was going as high as 300+ mph under canopy and as low as 30 mph in vertical freefly.
Also, I would like to say that I have never dowloaded any new software from there website. My review is on one neptune straight from the manufacture. (I have heard that there is better software out there).
For the most part it has not failed me in freefall. I once watched the altitude reading get higher while in freefall. (Although it was when I was transitioning from back to belly in a track, which could happen with any altimeter because of the pressure differece). Also once in a while my neptune will remain in canopy mode throughout the entire freefall (which isn't a big deal if you don't mind reading four digits in freefall) Also, canopy mode can be turned off.
I have owned my Neptune for about three years now and have had to replace the batteries a handfull of times per year (and they are expensive around $5, but not difficult to find).
The audible function seems pretty decent but have not used it that much.
The clock on the neptune (in my opinion) should be taken off the list of functions. This is proved very quickly by asking the time in any crowd of skydivers. CAN NOT HOLD TIME! But who cares about time when distance from the ground is all that matters.
The MENU is great. I have never had to use the DZ setup. My neptune knows when i'm on the ground or in the sky. But has a Manual setup when the landing altitude is differn't from take off altitude. The Altimeters MENU also has a training altimeter for read out and audible alarms (which is fun when your in AFF or a newbie of anykind).
The backlight function seems to only work when pushing buttons(kinda like a cellphone) and dosen't have much of a night jump advantage. (let me know if there is a night jump type function).
Great accuracy. When I look at my neptune I trust the altitude reading.