It's difficult to get it adjusted just right and lock it in at that location. And of course the image quality isn't the same as what you see looking through the scope (unless your scope quality isn't very good).
But if you're just looking for a way to record what you're seeing in your scope and you don't mind that image quality will be mediocre, it would be fine for that.
I think I'll play around with my spotting scope and smartphone just to see how easy or difficult it is to get a good image and photo. Since you can see the image on your smartphone as you position it over the scope lens, it should be easy to determine how likely it will work. If the device holds the phone and scope together well enough, it becomes a simple matter of pointing the scope as you watch the phone screen.