Why not buy the cheap LED Christmas lights that you find in retail stores?
Because...
Their LED lights use a single diode to rectify the AC voltage to DC thus producing a half-wave DC output.

(As you can see the DC output isn't very constant and a flicker is produced.)
While our LED lights use a full wave bridge rectifier to produce a full wave DC output.

(In the animation above, the cycles are moving at 1/16 cycle per second, while in actuality, this is happening 960 times faster at 60 cycles per second.)
Also, an RC filter further smooths the output reducing the flicker even more. So to answer our original question, LED Christmas lights that make use of full-wave bridge rectifier technology are worth spending a little more for. Click here for a complete explanation of diode rectification.