There are so many gadgets for putting sharp edge on kitchen knives these days that it's often hard to pick a good one. In my experience, most of the "easy to use" approaches are easy because they don't do a very good job. When it comes down to it, if you want a really decent edge on a knife you should be using a whetstone.
This one is a reasonable choice, being very competitively priced, of a decent enough size to cope with an 8" cooks knife and offering two levels of coarseness to first get an edge and then really hone it sharp.
It's supplied with a plastic/rubber "foot" to stop it from slipping which is very useful as you've got to soak it in water for a few minutes before you use it. Simply hold it on the bottom of a basin full of water and once the bubbles have stopped, you're good to go. It's the slurry of water, grit and metal that gives a whetstone it's sharpening action so a good soak is essential. This does mean that it's a bit messier and more time consuming than other methods but it's worth it.
Sure there are coarser/finer/more expensive whetstones available and if you've got REALLY expensive knives you might benefit from them but if you've got a set of reasonable kitchen knives then this will do a good job of sharpening them. My "supermarkets own premium knives" sharpened up a treat using this stone and the edge was good enough that they stay sharp with a few occasional strokes along a steel so I leave the whetstone and it's wet, slushy paste for periodic reinstatement of the edge.
At this price, I can't think of anything I could usefully improve - for that it gets 5 stars.