The cheap DX/Kai lights tend to be more erratic in build quality - sometimes you get a really nice light, sometimes you get a real dud where the switch flakes on you, or the circuit blows out. The waterproofing tends to be not as solid, either.

The Fenix lights are more consistent in their build quality, with good customer service and fancy features like multiple modes, etc. Then again, they're more than $50, too. For myself, with a bike application, my sweet spot lies somewhere in between. On a bike, I don't want all the fancy modes, but if I can find a well-made light for about half the price, then I can get two of them.