At Minigore 2 I did really cheap rain effect using couple hundred simulated particles which was each rendered as mesh that contained around dozen actual raindrops. This keep fillrate quite minimal. That reindrop bunddle Mesh was duplicated to same VBO 20 times so it was possible to render 20 particles with one rendercall. Each particle was frustum culled and tested against y = 0 plane. If collision to ground plane happened watersplash effect was spawned which used quite similar pseudo instancing trick. Particle was re-emitted after collision and emitter cube followed camera. Whole effect used around 10 render call without any dynamic geometry. Rain was added just before release so we didn't have that much time to fine tune the effect. It's actually even use snow flake texture and was vertex colored to hide this fact.
Edit: the effect start from 1m6s. Tried to link straight for it but did't quite nailed it.
That looks really awesome!