Rye adds a nice spice note as well as some viscosity. Don't go too high, unless you want to make a Roggenbier. Although the flavor is appealing, adding too much will leave a cough syrup consistency. I have made 50% rye beer and although it was tasty, it was hard to get over the thick mouth feel. A Protein/Beta Glucan rest can reduce some of this when rye is used in large amounts, but I have never been able to eliminate it. 10% or so in the grain bill is about right for a hint of flavor without all the extreme effects.