|The Saliraptor, which comes in black for crafters and white for adventurers.|
On the ground, including in areas that are not yet flight-enabled, leapers are slower than regular ground mounts. The difference, in flight-enabled areas, is that they can jump. Really high. Like on top of buildings and up sheer cliffsides high. The leapers also come with immunity to falling damage, which means that you won't kill yourself by leaping off a cliff or jumping towards a bridge and missing.
It's an interesting mechanic - the jump is high and long enough that you can basically avoid combat almost as well as if you were on a flying mount, but this method takes a bit more attention to ensure that you don't fall off narrow landing spots, get stuck under terrain, etc. Touching down on the ground also helps spot quest items, some of which aren't visible from too high above the terrain.
(The level 60 version supposedly adds a slow-gliding option that increases your travel distance and control. I can see some value in this - when the leaper hits the end of its forward momentum, it basically drops straight down like a rock, in a way where a slow-falling mount would manage to coast across.)
I've never been that fond of player-controlled flying mounts - I think that what we give up in terms of avoiding/trivializing content is not worth what we get from the experience of flying over the world, and I think that 99% of the benefits can be achieved with either NPC-controlled flight or player-controlled flight in specific constrained areas where being able to steer actually contributes to gameplay (e.g. airborne bombing runs). As far as compromises go, the leaping lizard is not bad.