I think this is a really bad marketing situation caused by the LCD and calling it a "back button" attachment. At first, I thought this was a joke, like Sony were releasing a new attachment with a giant button on the back of the controller just so you can go back to a previous menu with the press of a single giant button.
The fact is, this isn't a "back button" add-on, it's a paddle add-on, and it's a great price. This will give you two added buttons on your controller, and it's an official Sony product. I've been waiting forever for something like this. Finally, another company released a similar product about a year or two ago, and I've been using that ever since. I can't tell you how invaluable paddles are in so many games. If you don't believe me, ask anyone who has ever used a scuff controller, which costs about 4 times as much as this does.
We also don't know how much more this will drain the battery, or even if the screen will be on at all times. It's also not like the screen is going to feature a 1080p resolution on the back of a controller, on a circular button for 30 dollars. It's just designed to show you which buttons you've mapped and what profile is being used. It might not even make much of a difference in the life of the battery.
Anyhow, this is what I'm using currently, and I wouldn't ever want to play a game again without it, or something with even more paddles: