It was good for the time. So were java applets... but, as they say, to everything there is a season, and the sun is season is changing in the web media space..
It's really not comparable. Flash + AS was a RAD environment, many a game developer got their start (and even built full-fledged games[0]) through it. I'd argue that its biggest contributions were not web-bound[1], and I don't know of any other environment which is both as easy to get started with and as flexible.
[0] it's famously the primary expressive environment of Edmund McMillen: Gish, Aether and Binding of Isaac are all Flash though the later got rewritten natively in Rebirth, Super Meat Boy is the native successor to the Flash Meat Boy, The End is Nigh may well be his first game which did not start in Flash (and even that's unclear as it grew from a 2-weeks game jam)
[1] except insofar as they were being embedded/distributed through it e.g. newsgrounds or armorgames
