There are many posts already on this forum discussing Leigh dovetail jigs. A search for Leigh + Dovetail entered this way will likely bring you more reading than you can handle.
I have the Leigh D4R. One of the reasons that I bought it was to get the 24" capability because I frequently make drawers that are 8-10" high. Having a 24" wide jig lets me set up and cut both drawer sides with one setup, since the dovetail patterns on the two sides are usually a mirror image of each other with a wider tail left for where the drawer bottom dado gets cut. I can complete a drawer without having to change the setup between doing each side.
The Leigh Dovetail jigs work best with a fixed base router that has the ability to use the Porter Cable style guide bushings. Leigh provides the correct bushings in this style with their jigs. I have found that having 2 routers makes cutting dovetails easier, since one can be set up with the dovetail bit and the other can be set up with the straight bit. Getting the exact bit height is critical for cutting good dovetails and I don't like to loose this setting when it comes time to switch bits. Having 2 routers lets me avoid this. It's possible to make dovetail joints using smaller jigs and with only one router, but it takes more setup time and you are forced to make your parts in batches, all of the right, then all of the left, all of the pins, and all of the tails, etc.
I'm very happy with my dovetail jig and the precision that it can achieve. Leigh products do everything that they say they will do, if you follow their well written manuals and training DVDs.
Central North Carolina