A must have for people that have to play D-tuned (pun intended) on a regular basis. To first setup both tunings can be a bit long, but once set, it won't move.
Note that the way it works you actually chose a tone difference for both tunings, not an absolute note, meaning that you can set it to range anywhere from a half step down to two full steps or maybe more. EG. you can set it to drop from say E to D#, D, C# or even C if your string gauge allows it.
I've been using it for more than a year now on my main bass and I'm more than happy with this product. Besides its practicality it alleviates the strain on the E string that a non stop tuning from E to D brings.
The only downside that comes to mind is that the diameter of the pot where the string is coiled onto is a bit big, so the main tuning key itself is quite sensitive, sometimes a bit too much for my taste. But thanks to a great build quality the tuning doesn't move all that much anyway.