Koa, as the man says, is characteristic in its tone qualities. It has some mahogany qualities -- brightness and "zing" -- and some rosewood qualities -- complex overtones and "deepness" of bass -- and is really a personal choice among players. That is, some hate it, some love it, and some are ambivalent.
I think of it as a "chimey" version of rosewood, or as a mid-range-heavy version of mahogany.
It's used for ukes a lot, too, and you might get an idea of its qualities by listening to tenor or baritone ukes (which are larger in body than soprano ones, and thus closer to what a guitar would sound like). I have a couple of Koa ukes (one tenor and one concert), but have never gone out of my way to get a Koa guitar. Probably because I don't need that sound, prefering my mahogany Martin or my sycamore (a relative of maple) Running Dog, or possibly because I can't afford one.
Whatever, Koa can be an interesting choice for a guitar tonewood, but you need to play the "target guitar" to make sure you like it. Remember that the maker and model will determine how much you like it, too -- the first major maker to make Koa a large part of his run was Bob Taylor, and his company makes a wide variety of models using the wood, so you're not restricted to just a few dreadnoughts.
Koa makes an interesting top as well as back & sides, too -- with better results than mahogany, which Martin used for lower-level guitars (the 00-17 or 00-15, for example) "back in the day." All-Koa guitars are pretty desirable, it seems (to judge by the prices).
Like I said, it's a buyer's choice thing, but don't be afraid to try 'em. Many do, and many like 'em.