This is a very flexible device and a tremendous help when you need to cut or slice a lot of vegetables. Shown in the photo is the Oxo 1054752. Not only does this device slice to any thickness you wish to set (up to about 3/8 inch) but it will also julienne, cut fries, and do ripple cuts as well as straight cuts.
Keep one thing in mind - the Mandolin is a very dangerous device which should be used only by consenting adults who are not currently impaired by drugs or alcohol, and kept completely away from children and teens. Accidents with a mandolin don't just cut, they commonly take a chunk of flesh and heal slowly.
All mandolins come with a safety slider (the hat shaped item lower left in the photo). It can't always be used, such as when you start slicing a cucumber, but always use the slider when you get near the blade. I's a bit clumsy at first but you need to get used to it to prevent accidents.
I confess, this is just about impossible with something like a Persian cucumber. For things like that I slow way down when nearing the blade, and do the last few slices with a vegetable slicing knife.
More on Knives.
Note that to slice tomatoes you need a "V blade" slicer, not a straight blade like the model shown above. I always us a knife for tomatoes so the straight blade works fine for me. Oxo also makes a V blade slicer (1155700) which is highly thought of and less expensive (US $35 to $44 vs. $60 to $78 for the 1054752).
Another very popular V blade mandolin is the Swissmar Borner, priced similarly to the Oxo V blade (US $35 to $40). It is a bit limited with just a few pre-set thicknesses available. It is said to have the easiest to use safety slider.
The Oxo I use has been criticized for the blade not being sharp enough, but it is easily removed and sharpened. Some mandolins have blades that cannot be sharpened or can be sharpened only with difficulty. This is something to look for when selecting a model.
One feature I very much like is the sturdiness and stability of the Oxos. I understand the Borner has no back feet so it needs to be held up with one hand while being operated with the other. This seems to me rather clumsy.