I brewed 2 buckwheat beers - an ale and a lager, both with kasha (next one will be brewed with flaked buckwheat). The buckwheat aroma is very strong, nutty flavor, somewhat grassy. If you are looking for this flavor & aroma, go light with hops, perhaps not more than 25IBU. If i can suggest something, use hops that give softier bitterness than Marynka or Magnum, like Perle or Sybilla.
Other buckwheat-based adjunct is flakes, much easier to use (does not need long gelatinization). Nutty flavor from flakes is rather faint, no difference in aroma. My next buckwheat ale will be brewed with 25% of buckwheat flakes. The problem with kasha is that it has very high protein content and should be added to the mash at the beginning of protein rest (additional time to cool the kasha, as the gelatinization requires temperatures above 80*C).
All that is about roasted buckwheat (either as kasha or flaked). There's also white kasha buckwheat, which gives very soft, silky flavor similar to unmalted wheat. I can imagine it as a part of witbier or some other light, easy beer.
Anyway, i'm pretty sure you'd find more information on buckwheat use (and recipes, of course!) at our Polish HB forum.