We did this on our third day and really were glad that we did. We did the 1-hour tour and got the free DVD for booking online. (By the way, the DVD is worth it if you book online and get it for free, but not for the $20 or whatever you had to pay for it, it's only like 20 min. long and doesn't have that great of quality, but was a nice gift for us since it was free!) This was something my husband was adamant about doing and since I get really motion sick, I wasn't originally too keen on it. In fact, he originally started wanting to do a doors-off helicopter ride and I was excited to find Air Ventures had such great reviews so that I could suggest this as an alternative to the helicopter, which went over well and I think he's very happy that we did it this way.
You really get to see the entire island from above and get some of the history from your guide. Our guide/pilot wasn't as talkative as we would have liked, but really no one on the plane was asking questions or talking to him either, so I think it's understandable that it turned out that way. And most of the sights speak for themselves. You get to see the coffee fields, all of the fabulous/famous beaches of the island, waterfalls, Waimea Canyon, where Jurassic Park/Raiders of the Lost Ark was filmed, etc etc etc --- it is really all encompassing and I would recommend it.
The people that run it and were the pilots were SUPER nice as well, which really makes for an awesome trip, especially because I was a little nervous about flying in such a small plane.
As for the motion sickness, I did ok for a half hour and then was feeling really ill for the second half, but I never actually got sick and imagine it's MUCH better than a helicopter ride would have been. And they were very sweet about it as well, offering watering and where I could sit to cool off, so that was much appreciated.
I think this is a must do for those on Kauai, it's so cool to see all of the places from above and then spend the rest of the week exploring from the ground or water, very cool!