Looks great! The biggest drops sort of spoil the illusion at times because they pass in front of or behind other drops without absorbing them. Maybe a cool tweak for v2 would be to add some simple collision resolution? It would probably look great if you did a front to back sort based on droplet size, and resolved collisions (which you'd have to detect) by deleting the smaller droplet.
Looks like they did: http://maroslaw.github.io/rainyday.js/demo012_3.html
That patch of blue sky on the right doesn't feel quite right and takes the realism out of it a bit.
I know, just me being fussy. Impressive use of canvas though, it could be really nice as an appeasing background image on a site. Using the navigator geolocalisation and a weather site we could have a weather sensitive site. Have to test resource wise and see.