My first really good edges came from a Lansky. A lot of that had to do with finally mustering up the patience to sharpen any knife with any system or tool. That was the biggest lesson learned for me.
Having said that, I've used the Lansky, Gatco, DMT Aligner (Magna-Guide) and the Sharpmaker. They're all very good, even excellent, at their intended jobs. The guided sharpeners are best-suited to re-bevelling. I've generally used them only once or twice for each knife I've done, then all my maintenance is done by other means (stropping on bare & compounded leather, and also 'stropping' with very fine sandpaper). Occasionally, I'll give an edge a few, very light passes on ceramic (Spyderco DoubleStuff or the Sharpmaker). When PROPERLY utilized, the Sharpmaker, in particular, produces an excellent edge on knives needing relatively light touch-ups (anything short of re-bevelling).
Particular advantages of each of the guided sharpeners:
1.) Lansky: the clamp is probably the best of the three, allowing more angle options. This clamp is also better suited for smaller blades, it doesn't get in the way as much. I've even rebevelled the pen blade on a SAK with it; haven't been able to duplicate that with the other clamps. The hones are small, but this can be advantageous on small blades (pocket knives) and also for doing recurved blades (narrow hone fits inside the recurve better). Uses standardized screws for the clamp, which makes it simple to swap out longer screws for thicker blades. The method of attaching the rods to the hones, with the short end of the 'L' extending upward into the hone, makes it possible to adjust the rod flush with the face of the hone, as the hone wears over time. There's no offset between rod & hone to complicate setup.
2.) Gatco: the hones are larger than the Lansky's. Obviously better for larger blades, more metal removed per pass. The larger hones, combined with the relatively long rods, make for a longer 'reach' with each pass, too. The Gatco also uses standardized screws, same size as the Lansky in fact (10-24, if I recall).
3.) DMT Magna-Guide: for me, the easiest to use 'free-hand' i.e., without mounting the jig to a post or in a vise. Makes replicating a free-hand motion easier, when used with the Dia-Folds. The DMT diamond seems to cut much more aggressively (faster) than the diamond of either the Lansky or Gatco. DMT's 'mono-crystalline' diamond really seems to make a difference there.