I received a pair about a week ago. The female spends quite a bit of time swimming out in the open, but the male spends 99.9% of the time well hidden at the bottom of the tank or deep in some java moss. I generally have to spend several minutes just to catch a glimpse of him if I can see him at all. This is in a heavily planted tank with a handful of neon tetra as dither fish to allow the Dario Dario to relax. I'm hoping with time the male will relax enough to spend time out in the open, but we'll see.
They are very small fish that tend to remain quite stationary the majority of the time. Their movements are very deliberate and they seem to spook easily by larger fish that move quickly. They need lots of hiding places and you should really only keep them with other small fish that are not aggressive eaters. If I can remove the tetra from the tank without having to destroy the scape, than I will as they seem to eat most of the available food. I think they would do best in a small species only tank. Also, you don't want to keep more than one male unless you have enough room and hiding places from what I have read.