I have been using the iGills as a backup computer for about 6 months now and for the most part I love it. I typically dive 3 or 4 times a week, so I am using mine more than most users. Every great once in a while mine will have an issue with booting up before a dive and I have to undock the iPhone and re-dock it, but other than that all is well.
On night dives I would rather use the iGills than my main wrist mount computer because of the way it lights up very brightly. There is a daytime and nighttime setting which changes the display colors of the screen making it perfect for night dives.
The built in gyrocompass is more accurate than my traditional liquid-filled compass when diving around all of the lava rock here in Hawaii. The minerals in the lava rock do not seem to mess with the iGills compass as much. With my regular compass I have noticed that if I move it near the rock sometimes the reading can change by as much as 10 degrees or more.
One thing I have noticed is before your dive and on your surface interval make sure to keep the iGills out of direct sunlight as it can overheat the phone causing it to shut down.