Buy it. Now! (Just kidding)
My son has Google Cardboard (well not the actual one but a similar one made from a kit). I just got Gear VR for my Note 4. There is no comparison between the two experiences.
Gear VR is so much better in terms of visual experience, smoothness of movement, the visuals, available apps, comfort, everything.
The big thing that helps the Gear VR is the Gear VR hardware, specifically the head movement tracking. It's not just a holder for your phone. It's so much more.
Listen to this podcast at the 15 minute mark. They talk about the difference between Google Cardboard and VR. It's night and day.
http://pca.st/1bXB
The podcast talks about the IMU (inertia measurement unit) difference between a smartphone using Google Cardboard vs the IMU in Gear VR. The Gear VR is so much more fluid and smooth than Google Cardboard because of this.
So I'm saying all of this to let you know that testing VR in Google Cardboard and trying to figure out if you want to buy Gear VR based on this experience only, is like trying out a bicycle and deciding if you want to buy a high-end sports car (a Lamborghini). There is no comparison.
Maybe that's not a good comparison but don't let Google Cardboard be the only thing that you use to base your decision on.
I hope you can test it before you buy it. If not, rest assured the Gear VR experience is very immersive and awesome. I highly recommended it.