This has been covered a million times, but I'll summarize.
1: It's utterly useless for re-profiling, repairs or anything that requires material removal on a serious scale.
2: It is great for keeping knives that are already in good shape sharp.
3: It is easy to get good results with, as long as your edge bevels are set at the correct angle to use it.
4: You can improve performance for material removal by clipping or taping wet-or-dry sandpaper tightly around the rods.
I have a Sharpmaker, EP, WEPS and a belt grinder, and the Sharpmaker still sees bench time, despite the other three costing at least three times as much each. It's a useful tool, just not the ONLY tool you will need if you have a knife that is severely neglected or has bevels that are set at an angle that's not between 30 and 40 degrees. If they ARE set in that range, it's hard to beat for keeping them useably sharp for a long time!