I think the "flickering" is actually intended, I think its quite realistic ... I think its an observed phenomenon: when a drop is in motion down a pane of glass with bright lights behind you often see some fairly drastic changes in which pin-points of lights it's refracting into your eye at anyone time, resulting in a sort of rapid flickering inside the drop.
There are a few small issues:
* There is a lot of flickering of the raindrops
* When two raindrops collide, they do not always merge correctly - sometimes the size is wrong, or the position is wrong