Nowhere near as gross as melon! It's not unpleasant, sort of sweet, a bit kiwi-ish. I think some people would consider it to taste of nothing. Would be a good palate cleanser. It's refreshing, but not too sharp or sweet.
It's quite hard to describe somehting that doesn't taste of much!
I think melon is tastier, but I am one of those people pigletion referred to - I think dragon fruit tastes of nothing, pretty but expensive for not tasting of much!
I admit, I'm a melon hater. I was given it as a starter at a friend's house and out of politeness ate it, I swallowed the chunks whole like tablets with my glass of 7up!
I agree with you about price, although they seem to be on special at the mo, they don't taste enough of anything to justify price (though they are pretty!). I'm sure they would be good as refreshment in a hot country and probably more tasty. You could try and grow a cactus from the seeds...