I have many many Arkansas stones. I began with these and progressed to waterstones and overall waterstones are much faster. But, as previously mentioned there is some overhead with getting the stones out, soaking them, and dealing with the mess around and in the sink so I have recently been using the Arkansas stones again. With some good technique they will sharpen any knife (steel) I have, it just takes a little longer. I find getting my lily white Washita and a hard Arkansas will bring any knife I have that is not super dull back into shape pretty quickly. But if I have a really dull knife I will pull out the waterstones, there are better at moving metal.
One other reason for the Arkansas stone is I found with my pocket knives I would gouge the waterstones more frequently so for these I almost always use the Arkansas stones.
+1 For the slow cut, great to learn on though, it is hard to do a great deal of damage with them.