I have used one, and have no trouble shooting out of it. The key is you have to lean your hips/thighs against the rail. If you have it adjusted to where the rail is about waist high, then this allows you to bend at the waist and shoot with zero issues. I've never seen it get in the way when trying it this way.
However, if you try to remain close to the tree for a shot, then yes, it will almost always be in your way...
My favorite stand is the Summit Cobra (that exact model is no longer made) but is has the full arm rests for comfort, but is an open front. Even still, if you remain close to the tree those arm rest risers will be in your way as well unless the animal is directly in front of you.
I like the viper - like I said, you have to set is "high" and you have to be against the rail when you shoot.