You can learn about correct angles for sharpening each type of knife and each kind of steel, as I did, but that information is only useful if you know how to consistantly apply it to a blade. The key word here is consistancy. Can you repeat the operation with the same conditions, thus producing the same results?
Unless you are a practiced and experienced blade sharpener, you won't be able to put a consistant edge on a blade free hand. If you decide that the blade you're working on needs a 10 degree relief and a 15 degree edge, how are you going to know that you are actually doing that? And, more importantly, how will you know that you are duplicating those angles the next time you sharpen the same blade?
The sharpening jigs listed below all have angle gauges that appear to solve those problems, but they all can suffer from the same problem if you lack one important piece of information. The actual angle you achieve is determined by how far the EDGE is from the base of the jig, thus the angles on the jig are only accurate at one specific distance from the base. This isn't a design flaw in the devices, it's just a law of geometry - shorten a leg of a triangle and you increase its angle. If you don't clamp the blade at exactly the same distance from the base each time you sharpen it, you will get a different angle each time. And among a variety of knives, each blade, being different dimensions, will give you a different angle unless you use "base to edge" as your measuring and clamping standard.
Now mind you, the actual angle is not that critical. If it were, nobody could get a "proper" edge by eye. The important issue is consistancy. Each time you sharpen the same blade, you want to retain the same angle. This requires clamping in the sharpening jig in exactly the same way each time you use it.
If you clamp the edge farther from the base for your second sharpening, the stone will be riding on the angle where the relief and the edge meet and will do nothing to sharpen the edge until you've removed a lot of metal.
If you clamp the edge closer to the base at the next sharpening, the stone will ride over the edge and start a third angle. You'll end up with a rather odd, multi-angle edge that will not be as sharp as it was after the previous sharpening.
Both of these errors can be overcome by simply removing enough metal to form a new relief or edge. But under most circumstances, the goal in sharpening is to achieve a fine edge while removing as little metal as possible, thus prolonging the life of your blade through many sharpenings. You can sharpen by taking off lots of metal, but you will wear out the blade prematurely. Compare these two pictures of the same model knife. Note how the upper one has had a LOT of metal removed, probably by many, many sharpenings done more or less poorly.
As long as you understand this rule of geometry and apply it to your sharpening jig, any of these manufacturers' products will serve you well. They all work on the same principle and can be used to put a good edge on a blade. I own a Gatco but would buy an Edge Pro Apex if I could justify the expense. It's about 5 times the cost of any of the others. The advantage of the Edge Pro is that it's continuously adjustable. A video on their site shows how to match the existing angle on an edge by using a felt marker like machinist's blue. That way you don't have to actually know the angles and can successfully sharpen any blade to a reasonable degree without being an expert.
DMT
Edge Pro Apex
Gatco
Lansky
Razor Edge Systems
Smith's
There are others as well.
In my web surfing research, I found an evaluation of some of these kits. The reviewer stated that the angle markings on the jigs were consistantly off by a given amount.
The Lansky was 3-5 degrees low.The reviewer failed to take into account the geometry rule that determines the angle. Having watched the videos on the Edge Pro site, I can see that it is the easiest to set up for consistant base to edge distance and thus consistant, repeatable sharpening angles.
The Gatco was about 6 degrees high.
The Apex markings were accurate.
Now, having said all that, there's another issue with these sharpening jigs but it's only for those truly obssesed with picky details that don't really matter in the real world.