I'm fortunate to have tried 3 different iterations of the 7th lever control and I'll give you a summary and my conclusion. I first added the lever to the back of the old transmitter and updated everything except the gimbal firmware as outlined by another member on this forum. This allowed "absolute" gimbal control. The movement of the tilt is only as smooth as the continuous movement of your finger controlling the lever. For me, this was only easy to do when I wasn't actively using the controls to fly the quad. Works great with GS, hovering, or when you're moving in only one axis. Otherwise, the tilting motion ended up not being as smooth as I would like since it was difficult for me to smoothly control the tilt while controlling the other sticks. And since the control is mechanical, there ends up being a bit of slop in the lever control no matter how much calibration you do. I also tried the "relative" gimbal control that you get after updating the gimbal firmware. The movement of the tilt is much smoother as you can see in Mako79's video. But as I found out and is evident in the video, though the tilt is smooth, it isn't the easiest to stop the tilt when you want since you have to find the neutral point. Many times, you ended up slowing the tilt only to tilt in the opposite direction. Between these two methods using the old transmitter, I prefer the "absolute" since the tilt stops when you stop moving the lever and the smoothness of the tilt is only limited by how smooth you can move your finger controlling the lever.
Now on to the new transmitter. In my experience, the difference is literally night and day. It combines the silky smooth motion of the "relative" control with the accuracy of the angle of the tilt from the "absolute" control. I also prefer the location of the dial on the new transmitter vs the placement of the rear lever on the old transmitter. For me, it's just easier to control. Since I only bought one new transmitter for one of my quads, I've kept my other quad with the "absolute" control using the old transmitter. The difference for me is so great however that I'm considering buying a 2nd new transmitter.
